It doesn't have to be from GOG, but it definitely has to be old (pre-2001 or around that period classifies as old I guess).

Mine is Redline. I've waited 12 long years before I finally got to play it. Finished it in two days. What a stupid, dumb shooter, with very clunky on-foot mechanics, but what a lot of fun it is! Not as good as Interstate 76, but still very cool; from another POV, it's rather more like the stunted little brother of Interstate 76, but with a prettier face.

What's yours?

Currently playing Return to Krondor :-)

Fallout.

It's comparatively old to my age.

Centipede!

I just finished Realms of The Haunting.

Depends on how we define old, but on my now playing list is:
Final Fantasy II
Final Fantasy VII
Syberia
Painkiller
and I just started a NSG-run on Final Fantasy X

Fake-edit: Fuck, I'm playing three Final Fantasies at once? This isn't gonna end well...

I think it would be Softporn Adventure, the 1981 precursor to Leisure Suit Larry.

King's Bounty

I actually just played that a while ago too. It's delightfully juvenile. This is a better link.

I played and finished Another World over the weekend. I also played some Super Mario Kart splitscreen.

How can you tell the difference? Same story right?:)

TA: Kingdoms, it was about as dull as I recall it being, only made it a bit further in the campaign:(

Larry I

I recently replayed the first half of Thief: The Dark Project. I plan to finish it after The Witcher 2 and Fallout: Honest Hearts.

Runaway: A Road Adventure is from 2001 (original release) and 2003 (NA). Does that qualify?

I finally played the Icewind Dale games last week. I don't really know why I never got around to them when they were originally released. The almost complete lack of plot was unfortunate, and I think the only reason I finished them, despite having no narrative impetus, was so I could gain some closure on that era that I remember so fondly.