There are games that are freebies when registered, those are not counted.
First game is Divinity Original Sin, then 2 ... but it was via Kickstarter.
So I guess my first purchase via GOG is the Commandos 1-3 bundle
The first game I ever purchased on GOG was definetly "The Witcher". Yeah I mean the first game. A in my opinion not enough appreciated game.
Storywise pretty awesome but didn't age well gameplaywise. I cannot recommend this game enough for every Witcher enthusiast.