Driftmoon, maybe. I have quite a lot of RPGs from GOG, but this is the only one I would class as a "funny" full RPGs in my library. Maybe Overlord, too, but I don't think of it as a RPG.
I agree most of the RPGs are quite serious. Humour is present in titles like "Planescape torment" or "Jade Empire", but it's mostly as a break in a serious game, like the "funny sidekick" you'll find in many action movies.
Adventure games (i.e. point and clicks) are often far more lighthearted.
Outside of GOG titles, the funniest RPG I ever played was "Embric of Wulfhammer's castle", but it's ... special : Mild erotic game with "final fantasy 1" like graphics, very little combat, and a very good writing. The only game that had me laughting out loud for the past few years.
Post edited March 19, 2014 by Kardwill