The Witcher 2. I loved the first and only found out about GOG through RSS about the second.

Castles 1+2.

No games led me to GOG.

I either came across it by chance or read about it in an article.

I joined up for those free games but eventually stayed and bought some other games.

I found out about it from a friend. When I went to the site I could not believe the number of point and click adventure games, that were on my wishlist, were there. The prices are good and they work, unlike some of the old games I bought on cd's. Then I discovered the great community which was a real bonus.

Planescape: Torment

I created my account to buy Under a Killing Moon back in 2009.
It was a game I always wanted to play since I read the reviews back when it came out.
The following holiday season GOG gave away the first to Tex Murphy games (which I still haven't played) and had a great holiday sale.
I've bought many games since then, more than I have time to play unfortunately.

Edit: Oh and I first heard about the site on 1up's Retronauts podcast, back when they did announce GOG releases. And the podcast died and came back in a form a bit different.