"French" thread bottom bracket shells are also 68mm wide, and the fact that both sides loosen counter-clockwise means it isn't English thread (English fixed cups loosen clockwise). This pretty much makes the case for a "French" thread bottom bracket. And, as noted above, Nervar steer tubes (frequently used on French frames including Gitane) and often slightly oversize. Not to mention, previous owner could have sanded down a standard stem to fit into a metric diameter steer tube. The fork looks to be original to the frame, so try an English thread headset cup on the steer tube. I'll be surprised if it fits.