I'd guess a 40 year old Gitane would have a French threaded BB. Still doesn't hurt to check.

Assuming you know what way to turn it, soak the cup threads with some sort of penetrating oil, repeatedly, over the course of several days. Then, get out a propane torch and heat it up. If you keep the torch pointed at the cup, you shouldn't do any damage to the paint but take it slow to be sure. Get a big wrench and increase the leverage if needed by using some sort of extender. You can also try tapping on the cup to loosen up the bond between the cup and frame. The 'big bolt' technique should be enough to grip the cup but you will want to get a serious wrench to turn the bolt, preferably something with a long handle or a handle that can have an extender fitted over it.

I had a stuck fixed cup on an old PX-10 that took a week to get off. I used all the above methods and finally got it off but at one point I was worried I was going to crumple the frame due to the force I was putting on it to try and turn the cup.

The cup is stuck because the cup and frame have bonded chemically/galvanically/mechanically. You need to break that bond by using penetrating oil, heat, tapping, etc. Do this first, then resort to brute force for the final push.