Return to Krondor is awesome, as awesome is the Betrayal At Krondor \ Betrayal in Antara pack GOG offers! Don't let the dated graphics turn you down, BaK is one of the best CRPG ever.
Divine Divinity and it's sequel, Beyond Divinity are also often overlooked, and Divinity is seen as a Diablo clone, nothing more wrong! it's a great RPG with an interesting storyline and absolutely not only focused on combat. It has also a GREAT soundtrack, one of the best you'll find.
Stonekeep is great and a must have if you like first person dungeon games like Dungeon Master and Eye of the Beholder. It also has a book, telling you the first part of the story and setting a mood for the story itself, that sadly GOG never included in the release, but you can find it on the forums and you should read it before playing the game, it's really a great experience.
Arx Fatalis is another game I'd recommend, great stuff.
Another awesome RPG often overlooked is Vampire The Masquerade - Redemption, it was quite famous back in those days, but then Bloodlines obscured it's popularity. I think it's actually better (well, it's surely less buggy) than the sequel.
Leaving the RPG genre, I'd invite you to take a look to my GOGmix here: Unique atmosphere and gameplay
It includes many of my favourite games on GOG, some are famous, some are sadly overlooked