Here is a pic of our Bob Jackson which I restored and modernized.

Although this probably has a better tube set (Reynolds 531) than your Gitane I am sure you can still get your bike to go very well.
Contrary to what some of the others have said we find this old bike to be just as fast and enjoyable to ride as the modern tandems we also have.
Yes it is more flexible but I don't consider that to be detrimental to how it rides.
One of the main things with these old tandems is the short stoker compartment. Some stokers don't like it and some (like mine) don't mind.
On your bike I would,
-Set the rear stays to 130mm if they are less than this now. If you are a heavy team you may need to go even wider.
-Get a good solid set of modern road bike wheels.
-Convert to 9 or 10 speed. Chain, cassette, derailleurs, brifters.
-If you can't get satisfaction with the brakes change them. The alloy rims and new pads will help though. Also be aware that modern canti's and V brakes are designed for 80mm pivot spacing, yours may be narrower which creates some issues.
-I had to add some shifting pins on the chainrings to get satisfactory front shifting. You could either do this or put on modern ones if they are available for that crank pcd.