I am looking for some of the best Multiplayer games GOG has to offer. I am trying to bring my GF into the world of PC gaming. I tried with Divinity Original Sin 1 and I realized real quick that it was too much too fast for her.

It doesn't need to be split screen. She plays Mario Kart Switch and No Mans Sky. Thats it.

Trine 2 has coop mode and is on sale now.
Stardew Valley recently received a multiplayer update.
Lego games have coop mode.

https://www.gog.com/games?feature=coop&sort=popularity&page=1 Everything with coop tag on GOG. ;)

Lovers in a dangerous spacetime could be a nice choice. I haven't played it, though, so it's just a guess... :D

Terraria's multiplayer is glorious, and another shoutout for Trine 2.

If you're curious about the former, there is the infamous Jesse Cox and, sadly, deceased Total Biscuit's terraria 'coop' videos : https://www.youtube.com/watch?v=89fW4MTJvRs

Starbound offers similar gameplay to Terraria, with story and quests. You don't start on the same planet, but you can teleport to each other (depending on difficulty setting and where they and you are), and it shouldn't take very long to get interstellar drive.

Dying light

Me and the wife have racked up 100's of hours playing it

Haven't tried it myself, but I hope to sometime. It seems like a natural progression of the concept. My main worry though was what seemed like a frequent complaint at the time, that the world(s) seemed empty compared to terraria. Which didn't surprise me given how massive a game terraria is. But that was years ago. I understand the game has grown a lot since then.

Terraria is certainly a great game, and one I should dive deeper into, but for the moment I prefer Starbound, partially because of the various worlds built up in different ways and the mini quests you can get from tenants.

to bad you specified gOg only. Astroneer would be perfect.

Have you looked into Factorio