GFN is different from other streaming services such as Amazon Luna or, now defunct, Google Stadia. You connect your Steam, Epic, Ubisoft, Xbox and other accounts and, if GFN has that game on that platform, you can play it.
The advantage of that system - apart from the fact it's unique among streaming services - is that you can play (almost) whatever you want from your library. You don't need to play the games the streaming service planned for that month.
The shortcoming is that you must own the game you want to play on a certain platform, e.g. I have Elex but I can't play it through GFN because I own it on Steam and they stream that game only for Epic owners