despite the Campy DOs, if that 531 decal is original and correct this is a "3-tube" frame made with (maybe) plain-gauge 531tubing which was the spec for TdF bikes before they went to all-531. The forkcrown is also "more correct" for a TdF and there's no hiding that centerpull brake braze-on on the rear...also a TdF feature, not something you'd need on a Super Corsa or Super Olympic which should have sidepull brakes (and be ALL 531).
Gitane was famous (or infamous) for going off the rails with actual bikes that don't follow their published specs, and there have been examples of TdF frames made with Campy DOs...I think that's what you have here.
EDIT: Re-read your post and if the decals were added after a repaint, that 531 may be a red herring...plus the 26.4 would be correct for a metric 531 DB seat tube...I think a plain-gauge ST would be thicker walled and use a smaller post.
I withdraw my "TdF" verdict awaiting judgment from experts...
PSS: but that shifter-stop is not the right shape for a Campy shifter band...it IS right for a Simplex band...Gitane sure makes this fun!!