By watching a Belgian television serie called GR5 on Één I got inspired to start with hiking. But as an inexperienced hiker I don't know what I could expect from it. I've flowing questions:
The answers you seek are not easily quantifiable - for instance, a very fit experienced hiker might walk 50+ km a day on flat, open (i.e. not brush covered) terrain, but might only do 5-10 (or less) on steep, thickly forested hills. Some people can walk more than 100 km in a day - on urban streets/race tracks, but would be unlikely to sustain this day-after-day as you might on a long hike like the GR-5. I would guess that for an average hiker (assuming some fitness and a little experience) around 20-30 km per day would be reasonable with a weekend pack (food for 2-4 days, sleeping bag, tent, clothes, water, cooker). I've personally done easily double that on marked trails in New Zealand with experience and good fitness, but also had days off-trail where I've moved less than 1 km/hr.
Edited to add: A long long time ago (over 25 years) I was taught that with a weekend pack most people would walk about 4 km/h (2.5 mi/h) on flat terrain and about 300 m (1000') vertical/hour (i.e. to climb from sea-level to 1000 m (3300') altitude on an average hill would take a little over 3 hours). These numbers imply for 8 hours walking about 30 km (18 mi) on the flat or about 8 km (5 mi) climbing (that's a lot - Everest is 8848 m/5.5 mi high)
According to GRfive.com, the trail is 2500 km long and takes the average walker about 3.5 months to complete. It covers a lot of different climates, from lowland plains in the Netherlands to alpine passes in the French Alps. I don't know how you would spread out the days, but I suspect that you will have to stop a few times for the weather to be right - don't attempt a high alpine pass in a snow-storm! Being Europe, you most likely will never be particularly far from a village of some sort where you can take shelter if need be. There will be on-line resources and most likely blogs/vlogs which will tell you how people did their attempts.
Maps as mentioned in the comment by @TomasBy can be obtained from sporting stores, book shops, online, with GPS subscriptions, libraries etc.
I have no experience on the GR5 or anywhere in Europe, but I would expect that starting in the Netherlands (northern part) and working your way south would be easiest. This is so because the Netherlands is flat -gives you time to build fitness, and because it's in the North - colder, wetter, shorter summer. Conversely, the southern part in France is alpine - you don't want snow on the ground as an inexperienced hiker, or any risk of avalanche. You also want a longer window of time for conditions to be right to cross alpine areas - longer summer gives you this. In addition, jumping straight into the alpine parts with no fitness will be very challenging.
For training, I would start by walking. Go out 2-3 evenings a week for a walk. Build up to 1-2 hours if you can. On weekends, go for a longer walk. Build this walk up to several hours (take appropriate food, water, clothing). Now start doing it with a pack and some gear (you can substitute anything for weight if you don't want to get gear just yet). Once you can walk comfortably for a few (4-8) hours with weight over any terrain you can access easily; plan an over-night trip - carrying your gear!
As for the gear you think you need - ask a specialist hiking/outdoors store, not a generic footballs and basketballs store. At a minimum you will need a hiking boots/shoes, pack, hiking clothes (raincoat, warm clothes, socks etc.), food, water container, compass, maps (and knowledge of how to use!). In addition, as per @Jason comment - you should always have a fairly comprehensive first aid kit and the knowledge on how to use. What else you need depends on where you will stay and how - if camping you need tent, sleeping bag (down is best, warmest, lightest, most compact), sleeping mat, cooker. You should also consider a GPS and emergency beacon.
Most hiking gear of any decent quality is incredibly expensive, but buying quality is quite important if you are using it day-after-day - you really don't want to buy a cheap pack because it was half the price of a more expensive one, and then have it fall apart after a month of daily use, or not fit you well because the harness is poorly designed. Or for that matter, buy a cheap sleeping bag and be cold every night!
Pretty much as per @Bob1.
I would not start out on this GR without having experience (and good equipment) of camping 2-3 nights in a row minimum, several times. You should also be able to do a 15-20km walk with about a 25-30kg backpack and after having slowly built up the equipment.
You don't need to be a super-hiker starting out, but you do have to have built up sufficient fitness to carry the gear reasonable distances and you need to be experienced with what you need/want to carry with you. That takes a little bit of time, esp. sorting out needed gear. Why 2-3 nights days in a row? Because there are tons of things you can do without for one night out that you need longer time.
For example, you can stay out overnight with say 2-3 liters of water. But you can't carry water for a week, you need a filter/purification tablets for that. And I know I'd rather not drink purification tablet water for more than 2-3 days, so I would get a filter. You may think the opposite. Same thing with food. I find camping food extremely unhealthy due to the amount of salt in it, so I try to do without when I can. That takes a bit of experience and you'll rarely be too far from some form of small supermarket.
You get a very different perspective for what you "need" when you are carrying it day in, day out and when you have no easy way to make up for stuff you are missing.
On that note, sure, the GR isn't in the outer wilderness, but if you need something near a small town, you may not find a good camping gear store nearby. Take the time to get to know your equipment and since camping gear is very expensive, buy it slowly. Don't be too cheap on the equipment, buy less of it or buy good quality, but less fancy, gear. Bit heavier? Probably OK within reason. You could even buy used cooking utensils or anything not likely to wear out. On a long trip, bad quality gear will break down.
If you can rent or borrow a friend's tent for the first few outings, before you know enough to buy yourself, do so. Ditto for backpack. Sleeping bags and boots are not hygienic to borrow, so plan on buying that very early on. Plan for a mat under the sleeping bag, you lose a lot of heat without it. Keep a log/spreadsheet of what you carried and what you used. Figure out what you think necessary for 1st aid. That's one item you don't want to cross out because you haven't needed it yet.
Consider a solar battery charger, and if you like to read, an ebook. For maps, I would give serious consideration to a tablet/phone with a decent offline map app. Be careful with GPS: iPads for example only have a real one when it is SIM-equipped version. The cheaper models use "assisted GPS", i.e. looking at the local wifi networks. The GR is, probably (check out for yourself), reasonably well indicated on the ground. So a high-res topographic map @ 1/25000 or the like would probably rarely be needed, but it will take up space. Back the phone/tablet map up with a good paper guidebook for that GR with maps in it.
Last, before really setting out, I'd get a satellite rescue beacon, best a 406MHz one without fancy subscription/messaging but with built-in GPS. (the fancy messaging ones use a different, less reliable, wavelength). They're not cheap, about $200-300, but last for years and can save your life. No, a cell phone is not a rescue beacon in rugged terrain.
Tent? I always size it at +1 the number of occupants. 2 person tent for a solo. That's probably an extra 2-300 grams, but well worth the space. Tent setup time is a major consideration for me, esp when you have bad weather. Reason? Say you're hiking and you see a rainstorm coming. With a 5min tent pitch you can wait till the last moment to stop and take cover. Maybe it doesn't rain after all, so you keep going. With a 20 min setup you need to actively stop and start pitching your tent, just in case.
We already have two good answers pointing out that you should start small.
However, I would like to add another point--you also need to do shakedown hikes with the gear you actually will be carrying--including the max weight of consumables you will need.
I get the impression you have it much easier in the consumable department than we (I'm in the United States) do, but you need to check this in detail. I'm thinking of our Appalachian Trail, rarely will you have to go more than 50 miles before you can get food, but there is one section of one hundred miles where the only civilization you will see is logging roads.
Edit: A really extreme case: I just ran into a trail description here in the US that says for parts of it you might need 15L of water in your pack.
A lot of people tackle long distance walks like this in segments, at least until they're used to multi-day hiking. The opposite is through-hiking, but not many people can take several months off work.
The GR5 looks doable in segments ranging from a long weekend to a couple of weeks. Thus after a couple of local trips to test your gear and yourself, you can get a train to Hoek van Holland for a few days' flat walking with plenty of resupply/accommodation options. Alternatively, take a week or two of holiday and hike home from the start; this would be a solid introduction to long distance hiking, and an achievement in its own right. I assume you'll do it N-S.
Early sections could be done staying in hotels, meaning you could build fitness without having to carry full camping gear (or indeed as much water) as later in the trip.
Multi-day hiking in alpine regions requires more preparation, more water, and more kit. You can go from hot to cold several times in a day and have rather limited options for stocking up or sleeping in a bed. A full tent, cooking gear etc. is one option. Some sort of bivouac (possibly combined with hotels) is a lighter option, and you may decide to live on cold food away from villages to need little or no cooking equipment. Either way you need to test your gear first.
I've done a couple of weeks at a time of this sort of hiking years ago and it was great (Swiss Alps, some of the GR25). More recently, to give you a hint, I've cycled near the Briançon-Larche leg in June, starting cold at sunrise, drinking something like 8l of water and ending dehydrated in the evening. So detailed planning is necessary, and you won't know everything you need to plan until you've had a few mishaps.
There are lots of discussions here about maps, (maybe start here), but paper maps in reasonable detail are fairly easily available for most of the EU. You wouldn't want to carry much of the route at a time. I normally prefer hiking with 1:50 000 or even 1:25 000 maps, but the IGN 1:100 000 maps are adequate for GRs, which are marked fairly well, so long as you do you homework regarding detours to villages etc. I'd obtain the maps for the northernmost few days assuming you start at that end and don't plan on through-hiking.
I suggest that as a first step, you do some long day hikes from home in various weather conditions, solo. The solitude is not to be underestimated.
If you can read French the "topoguide" from the FFR (fédération française de randonnée, French trekking association) is a good option both for planning and during walk (I don't think they cover the part of the GR5 outside of France and you need several to cover all the itinerary). They have information on accommodation, food supply and transport around the itinerary, the itinerary is often described in time rather than distance, thus taking account for elevation change and ease of walk (the timing is for average walker, so a beginner will take more time) and the itinerary is described on a 1/25000 scale topomap extract so you don't need to buy an extra map.
I give this specific example because the question reference the GR5 but you will be able to find similar guides proposing maps, suggested itineraries and information on amenity along the way for most if not all of the "famous" trekking routes.
The hardest past of planning with this kind of book is knowing where you stand relatively to the information given (i.e. how long will it take you to walk this described 5 hours part? Are you able to walk 8 hour a day for several day while carrying a pack?) And as said in another answer, the only way to know that is to start walking...
I will give here some general guidelines for hiking:
Specifically for the GR5:
Then, how long does it take? It is really hard to answer that question as it will depend on how you want to tackle the route. It you follow the official stages, it might be 3.5 mounths. If you decide that you want a challenge, you can try to go faster. As it is a long way, it is quite easy to go faster as you progress; you should plan for shorter days at the beginning (20ish km) and then try to cover more. If you read about thru-hiking in the US, you'll see that they often average more than 35km/day.
I did the GR5 in the Alps and I was averaging 30km and 2000m of elevation per day. Then I discovered the thru-hiker mindset (basically: hike all day long), and now I average 40+km/day. Hiking is great because it's not technical: going faster or further is mostly a matter of motivation.
