"...That's really interesting mate, I'd never heard of Grande Randonee before, any particular routes/sections that you'd recommend?.."
Hard to choose really there are so many, better to surf the internet for pictures of the various French regions, find places that look like you'd want to be there and then search for routes in that area. However...
A nice meandering trail through undemanding (but pretty) countryside with perhaps the odd opportunity for stealth camping or various types of accommodation along the way and the chance to meet other walkers from elsewhere in the world, try a section of the
GR65, perhaps walking from
Mossiac to the Spanish border, that will take you through the
French Basque Country which is one of the most beautiful bits of France there is (IMHO), the folks are exceptionally hospitable and the food fantastic.
A slightly more challenging route that'll take you anything up to two weeks would be the
GR70 which follows the route taken by Robert Louis Stevenson when he walked across the Cevennes, s'étonner of this parish posted a trip report of his walk
here. Again the folks are very hospitable, the food very good, some opportunities for wild camping, a couple of long hikes, nothing that will kill a reasonably fit human being.
Hard core, you know the score, try the
GR 20 just over a hundred miles across Corsica, not an easy one but fantastic scenery, fantastic people, fantastic food. You cannot wild camp except near the mountain huts, you cannot have open fires, you need to be fit, you need to know how to walk without support for several days at a time if needed, you shouldn't think about attempting this in the middle of summer (or middle of winter) because you will die.
Lots more to choose from.
"...The GR10 starts (or ends) not far from my place in France.."
I wish I'd known I'd have offered to drop off some English/Scottish foodstuffs if you wanted (presuming you live at the western side).