GR34 is the absolute best, my man
As for your questions, I haven't been to the Pyrenees myself (damn southerners and their burning sun) but I guess you can find
http://www.pyrenees-refuges.com/#8/42.902/0.676 lots of empty stone cabins you can sleep in for free
lots of cool places to camp in the wild
good enough marking, but you still need a good map. IGN have some very good maps, plenty of options from 1/25000 to 1/100000 with varying cost. Careful tho, GR11 is the spanish name for the spanish trail, the parallel french one is GR10.