In reply to fshbf:
the camp grounds on the GR20 tend to be gravelly (sharpish) granite sand / pebbles; and peg-able (just) ground, rather than proper rocky.
the sand / gravel can be quite rough, puncturing your mat is a risk, but you should be fine if you're careful.
I had an exped synmat and managed not to puncture it, but somehow managed to get small wear spots in the tent grnd sheet (it was old).
not sure about exact temp (not worse than chilly in the mountains at night) - decent 2 season bag was fine for me (all appropriate disclaimers apply).
the main thing was rain rather than temp - get up early and aim to arrive before the afternoon storms kick in.......