Thanks for the reply, do you know which games here on GOG have this feature of just inviting friends? I can't play via direct IP because my Hamachi isn't working right now. Is there a way to know if a game has this feature on its page?

I know virtually nothing about online multiplayer, but isn't Hamachi mostly for games that require an intermediary service (like GameSpy) that no longer exists? Terraria allows a player to run their own server, and other players to join that game by connecting to that IP address.

With regards to which GOG titles have invite-based multiplayer (either through Galaxy, or through a service run by the publishers or developers themselves), I can't remember too many off the top of my head. Dying Light is one. I don't play online/LAN multiplayer, so my knowledge of which games have which multiplayer features is pretty spotty. I'm also not aware of any GOGmixes that track this particular thing.
I would suggest maybe creating a topic in (one of) the General Discussion forum(s) asking for info on which games have this feature/limitation.

You or your friend just have to port forward, to mine together. Third party softwares like Hamachi aren't required in any of your systems.
It's not difficult like it seems (there's multiple guides on Youtube and Google, have a look), and you won't have to rely on Hamachi every time you want to play with your friend.

To make sure you did things correctly, you can use websites like this, to check if the port is open or not.

I use a freeware I found many years ago, to check that, but the website above should satisfy your needs (if not, I can send you the download link. Just tell me).

I tried Hamachi once, and it was a waste of time and space in the HDD, tbh. Uninstalled it and learned to host a game the proper way, even if it was not the 'friendliest' option. Totally worth it, man.