I have played (and am currently playing France). I have survived two major plots to destroy France (E/G/I) alliance, and recently (E/R/T) alliance.
When on the defensive, I found that having an army in Gas was the most useful thing to have. I can actually attest that having an army there was the only reason that I recovered from both onslaughts.
I personally feel that
it is a keystone for a defensive stand.
Gascony borders all French supply centers. Good place for armies.
Gascony is the key to unlock France, and Burgundy is the gateway.
Gascony is at the very center of the French sphere of influence; it touches all three home supply centers and Spain as well.
If somebody else gets an army into Gascony, France is in big trouble because she doesn't know where the army will go and so has to defend three or four SCs. If France loses Burgundy and Gascony, the game is pretty much over for her because the enemy can support an attack into Par or Mar. The best way for Germany to take France is by land, and getting armies in Bur and Gas is an important step.