In my opinion:
The Witcher and Witcher 2 are actually pretty funny games at times, but probably not what you mean.
Fallout 2 when it was on here was hands down the funniest.
Torchlight, but mostly because I think it's funny watching the mooks run away as I shoot them in the back and they explode into modest mists of blood...
Arcanum does know when to laugh also.

Another vote for Anachronox, but another title with some humor is Baldur's Gate II with Minsc in the party. Takes a bit of the seriousness away.

Agree with these two but Brutal Legend is worth a shot as well (kind of an RPG). Funny stuff although the car mechanics may drive you nuts.

Overlord is great. Minions have surprised me into laughter more than once.

Baldur's Gate 2 has some really funny moments.

absotively posilutely!

Thank you for the kind words :)

ohyes!

Planescape and Bard's Tale are probably your best bet. Although I would count the Fallout series in this as well but ya they are gone now.

Not an RPG but there is this game, Flibble I got from a Bundle in a Box that I laughed playing, very funny writing.

Another but you won't get it, a French RPG Maker named "Vader ou la Fin des Haricots", has some nasty grounded jokes, like at the start, (Dark) Vader search his helmet and.. A Storm soldier did poo inside, with a splash screen and sound.. Lots of these, with recorded laughs behind.. Silly but hey, that's the break from serious business and it's free, but French stuff too much for foreign folks, I guess.

Edit : And sorry, skipped GOG in the OP request..

Ultima Underworld 1 has some funny and humorous moments. :-)

It's not on GOG, but it's available DRM-free via ShinyLoot: Grotesque Tactics.

The Divinity series apparently doesn't take itself too seriously without being excessively camp all the time.

Ring Runner: Flight of the Sages has some RPG elements and pretensions of being comedic, but it's one of those games that confuses references with parody. I'm not going to say that it's a bad game, but it didn't work for me in any way.

Neverwinter Nights 2 has at least one moment that probably comes across as random humour on the face of it, but since I picked up pen-and-paper D&D, I have noticed that the players really do things like that all the time.

I think Baldur's Gate I has very funny responses. And Noober. Noober is the deepest character ever, you must agree. xDDDDD

I think Ittle Dew might qualify. If you're willing to go off GOG, there's also DeathSpank:

http://store.steampowered.com/app/18040/

Definitely second Unepic and Overlord.