De plus, j'ai essayer une foto de nuit sur mon balcon, et il m'a pas semblé que l'appareil ai ouvert plus longtemps pour palier le manque de lumiere.... Des idées ?

Broken GSN?

Many GSNs haven't been used in over two decades. So it should come to no surprise that if you find a bargain GSN in a flea market, that it may not work. But often this is just because the internal contacts have oxidized from inactivity. Despite the gold contacts (the G in GSN), dust and evaporated oil can prevent good connectivity. A little exercise can often fix them.

Put the batteries in, then turn the aperture ring back and forth, back and forth multiple times. Also many GSNs have a very strange shutter trigger where the meter only turns on sporadically when you're pushing the shutter down. Try pushing the shutter button down veeeeerrrrrryyy slowly and see if the meter turns on at any point (you'll hear it more than see it). Do this multiple times until the contacts have self-cleaned and the camera should be fine afterwards as long as you regularly use it.