I's is not the actual possessive form. It would be like saying "ichs filmnacht" instead "meine filmnacht"

In English, the proper way to say it would be "my gf's and my annual bad movie night"

It's funny; when I was learning German, I thought genitive was so weird, but it honestly makes so much more sense than how we do it in English

#MakeEnglishGermanicAgain