Lets put our heads together, what games on GoG have cooperatyive play? Hopefully a couple RPGs.

Apache Vs Havoc has it
I THINK you can play Fallout Tactics in coop missions agasint the AI
You're able to play coop missions in Freespace 2 (I think also in number 1)
Thats about all I know. Most of the other multiplayer ones are adversarial

Doesn't Duke3D have coop?

Unreal games

Yes, it does.

Sacred has Co-op, and a few ways to go about it. Providing you can get it to work...

ah this post is a great idea :) yeah duke nukem is the only one i know if, i guess theres the unreal series but i dont think they were the campaign in co-op but instead just random spawning enemies or something like that... i could be mistaken of course

In Unreal 1 you can play the single player story in cooperative, but I think I might have some vague memory of the save game function not working. But maybe you can solve that by using some level skipping cheat code when you want to continue a game?

Unreal Tournament 2004 has a multiplayer mode where you just kill waves of monsters, and both UT games on GoG do of course allow you to play on same team against bots (but UT3 is the only UT game where you can play the story mode in coop).

One game that I'm personally curious about is Vampire: The Masquerade - Redemption. Has anyone tried that game in cooperative?

I had no idea the BG and IWD games had coop, sweet. Arcanum is another one with coop, though it's apparently fiddly to get working.

Well, if you want go the D'n'D path, I'd say user modules for NWN are a far better choice. They're usually not too long, there's plenty of them, and their MP aspect is really polished.

And Arcanum doesn't let you play the main story (unless there's a usermade module for that).