Starflight 1 and 2.

The Longest Journey and Dreamfall: The Longest Journey
It was a two-in-one bundle.

Stellaris and Witcher 3

Dungeon Keeper 2

Couldn't get my disc to work properly and heard GoG got old games working

They didn't even Support GBP back then, bought it in USD

Bastion
Pinstripe
Limbo

Technically, my first order was The Witcher Goodies Collection, a free bundle of bonus materials.

System Shock 2

I joined because i got the additional keys for Witcher I and II when i bought them on Steam. First ones actually bought here were Arcanum and Alan Wake though

Don't Starve.

Heroes of Might & Magic 1 through 4 (all in the same purchase). Followed by the Legacy of Kain series soon after.

Freespace 2
$ 5.99

Creatures: The Albian Years
$ 5.99

Sid Meier's Alpha Centauri™
$ 5.99


Roller Coaster Tycoon® 2: Triple Thrill Pack
$ 9.99

Ultima™ 1+2+3
$ 5.99

SimCity™ 2000 Special Edition
$ 5.99

Hyperdimension Neptunia ReBirth 1, 2 and 3

edit: actually I guess it was Duke Nukem Manhattan Project

(second purchase) Far Cry

I lost my physical library a while before I signed up on GOG, so I spent a bit too much and bought most of the series I cared for right off the bat even if for most it was third or fourth repeat purchase (wasn't hoarding exact copies, it was more like original language license, one or two good pirate translations, then legit but oftentimes worse licensed translation, and maybe the finalized gold edition to replace one of those if the game had one). So it's hard to tell, even if it was recent.

Profile says it's Homeworld series, but I distinctly remember that, while I wanted it, it wasn't the first, nor the tenth, nor even the fiftieth in the first purchase cart. And I got the free Postal even before that, but that wasn't exactly bought since I didn't pay for it.

Might be either of Deus Ex, Bioshock, Prince of Persia, Dishonored, Thief, or Supreme Commander series, or Planescape Torment and Jade Empire because they sure were somewhere about the first twenty, at least.
Might be Age of Wonders, Arx Fatalis and Aquanox because they're first alphabetically.
Might be humble Revenant or Alpha Centauri just because.

*looks in orders history*

Duke Nukem: Manhattan Project in February 2, 2010.