I would use the bottom of the cast in height measuring "ruler" on the seatpost as the minimum insertion mark. It looks like you are well below it, so only solution will be is to buy a longer seatpost ti get more insertion legnth. Your height would typically ride at least, a 58 cm sized frame. That Gitane looks to be a 54 cm?
BTW, welcome to the 84 Gitane TDF club!
Here's mine.....

I rebuilt it up to my personal preferred "Tout French" specs.