Can anyone suggest any good co op games that are on GOG?

The UT games are good fun with human team vs bot team, just adjust the bot difficulty to your teams abilites.
Also the rts TA that was just released can be good relaxing fun humans vs computer because the maps are huge and you can adjust the computer difficulty to play at your own pace.

Most strategy games can be played with 2 or more human opponents vs. the computer AI. Serious Sam is very fun if you enjoy retro shooters at all (massive waves of enemies and snarky one liners).
I don't see any co-op RPGs on GOG, but Din's Curse includes co-op (no DRM on that one) and Titan Quest was some seriously good fun in co-op (not on GOG at this time).
OH WAIT! I think Vampire: The Masquerade has some kind of co-op play option, please google it, it may be provided through the community (that game has a big one).