French bikes are great. Never owned a Gitane, but you can't go wrong there. Another possible method for determining tube construction is by measuring the seat post diameter. This, combined with the outside diameter of the seat tube (better check if it's metric sized) can give you an idea. The one caveat is that some makers put reinforcment inserts in the seat tube resulting in a lower number than expected. But if it's in the high range you can be fairly certain that at leas the three main tubes are of nice stuff.

Have fun!

Karl