I've been trying to invite a friend to play enter the gungeon with me as player 2 controlling the "cultist" character that shows up in co-op mode. we're both using GOG galaxy and own the game here.
Despite my attempts, after launching the game, the launcher does not allow me to "invite" my firend over, unike steam does with remote play or just invitations.
How do i play coop with my friends then? and whats the point of playing on GOG if it doesnt let me play with my friend?

Leaving this solution since i cant mark this post as "solved" somehow:

As for now, there is no way to co-op with people on steam or GOG, It has to be local if you want to play 2 players in enter the gungeon.
Steam simulates local gameplay by allowing one of the players to host the game with their "remote (share/remote) play together" (whatever the name is, i'm sorry i forgot it) so other people can join if invited.
GoG currently does not suppo[url=rt any of this fucntionality, so you'll have to either invite your friend to your house and play locally or buy and play it on Steam.

Hopefully, this functionality will be implemented to GoG Galaxy in the future.
]rt any of this fucntionality, so you'll have to either invite your friend to your house and play locally or buy and play it on Steam.

Hopefully, this functionality will be implemented to GoG Galaxy in the future.

You could always use a 3rd party solution like Parsec: https://parsec.app/

My friend and I played that way years ago, it was fun and worked well. We were both using PCs.
It has cross-platform too, so a person could even play off mobile if they wanted.