Can anyone recommend me a good RPG on GOG for LAN'ing purposes (co-op)? Looking for something I can enjoy with a friend or two.

I agree with WBGhiro, Sacred Gold is a really good Action-RPG for LAN co-op.
As for "classic" RPGs... unfortunately, it seems they mostly weren't intended to be played in MP.

now that i think of it there's also fallout tactics which should be a good multiplayer experience, but inever had the chance to try the multiplayer part.
GOG also list Warlords battlecry as RPG you may want to try that out too.

I don't really know how well Lionheart holds up in the "good" department, but it is classified as an RPG, and it does contain multiplayer. I know even less how good the multiplayer is, or how it works, since I never tried playing it that way.

It sure has multiplayer capabilities, but indeed, many a players don't consider it "good".
If dukejason reads this, he'll sure praise how great it is in MP and invite everyone to join him :-).

Thanks for all the help so far!
The size of Sacred is a little big for me, but I'll be buying it come pay day!

Wise decision! :-)
[url=http://www.sacredwiki.org/index.php5/Sacred:Underworld]Here[/url] is an exceptionally good reference site for the game, you may wish to read on characters (or other things) in advance, and it will come extremely useful once you have Sacred Gold.