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!