I only buy games when they are at least 75% off and at most $10 in my currency... I didn't buy any games for like 2 years or so, but early this month I bought Frostpunk, SiN, Red Faction, Mars: War Logs, and The Technomancer and already beat them, so I got these today (not feeling like playing anything that requires too much attention).
I have played Aliens vs Predator, Dredd vs Death, and FEAR before though.
Purely on GOG? It's been Fallout: London (one click edition, because lazy and Linux) and Sherlock Holmes: The Awakened (the remake)
The Sherlock game resulted in buying basically all the newer games in the series.
FOLON on the other hand makes me infinitely grateful for gog, as the f4 anniversary nonsense is kept at bay.