Heroes of Might and Magic 3 Complete, September 2011, at full price!

I might have been Master of Magic, Heroes of Might & Magic, or Temple of Elemental Evil. Getting those again was what excited me most when I discovered GOG. My physical copies are long gone and wouldn't run on a modern system anyway.
Oh, and original Stronghold too (and Crusader), perhaps. My first purchase was 4-5 games.

Terraria lured me to GOG. I blame the cute little bunny! Like Alice, I followed that bunny down GOG's DRM-free rabbit hole and been trapped down here ever since. ;)

In 2012, Slickdeals posted that Fallout 1 was free on GOG and that was my first introduction. Fallout 3 and New Vegas had been released years prior and a new legion of fans was flocking to the "classics" out of curiosity. Can't say that deal did much to pull me into the store but a few months later Duke Nukem 3D was offered for free. That did it. Searched the store, saw how many classics from my youth were offered, and waited for the right time to buy.

First game purchased was Roller Coaster Tycoon Deluxe for $2. then promptly sold my CD copy for $20. Gog use to do those great flash deals where so many units were offered and then another deal would present. Almost had to keep the GOG website up while at work during those events. Got some great deals too, Simcity 2000 for $2. Medal of Honor for $4. Leisure Suit Larry for $3.

Good times, simple times. The Zoom-Platform feels like that now.

Painkiller

Alpha Centauri, although I probably bought a whole bunch of games at the same time, knowing myself.

Darkest dungeon with a massive discount. P.s. really looking forward to the second part of the game

Populous: The beginning, which I still think is an excellent game.

I haven't played it for a long time, but last time I did, there was a reasonably active multiplayer and modding community at https://www.popre.net/. The game is quite innovative - and a lot of its ideas still haven't been fully explored by any newer games. So I reckon that still makes it worth returning to.

That said, like most old games, it's probably not easy to get excited about it without at least a small dose of nostalgia to get you started.

Definitely!

The first one was Fantasy Wars.

It's a good game in principle, but the turn limit sucks.

It was Legacy of Kain: Soul Reaver.

Titan Quest - Anniversary Edition and Grim Dawn.

Giana Sisters: Rise of the Owlverlord

According to my orders history, M.A.X. + M.A.X.2 bundle and Castles 1+2 (the same day).

But the game(s) that actually brought me to GOG were The Witcher 1+2, as I had heard you could buy them DRM-free here, digitally. That sounded too good to be true. Apparently it took me three weeks, after joining GOG, before I actually bought those two Witcher games. That was back in August 2011, oh how time flies.

A fun fact: I haven't properly played those two Witcher games yet. I've installed and ran them though, mainly to see how they run on my (former) PC...

I did try to play M.A.X. at some point but decided maybe it is not for me. If I recall correctly, it has a turn limit in its missions (it is a turn-based tactical combat game) which took all the fun out of the game for me. I decided maybe that game is not for me.

Master of Orion 2. I still fire it up from time to time ...