Here's a brief outline of our GR20 trip in June 2011 (‚??our' being me and my Dad). There are loads of threads on here so any questions about practicalities might be better added on to those: especially GR20 Blog Available
You'll see on that thread some of the questions I asked (including travel) before going and I'll be happy to assist where possible, though Paddy is normally on hand and is usually best placed.
We took Paddy's Guidebook, which it should by now go without saying is fantastic. We had no set plan for each day and adjusted our itinerary as we went to fit in some extras. We managed to take in Montes Cinto, Ritondu, and d'Oru andthe Aguilles de Bevalla. Unfortunately Monte Renosu had to be avoided because of extremely strong winds. Incredibly we didn't see a single drop of rain the entire two weeks; I appreciate how lucky we were. It brewed up a couple of times but never came to anything.