Well looky what I found...
http://www.gog.com/en/search/sort/genre/shooter/order/rate/price/999/developer
Scroll down the left side...
Developer:
* show all
* Altar Interactive
* Best Way
* BioWare
* Black Isle Studios
* Bohemia Interactive Studio
* Codemasters
* Computer Artworks
* Digital Integration
* Innerloop Studios
* Interplay
* Invictus Games
* K-D Lab Game Development
* Micro Forté
* Monolith Productions
* Outrage Entertainment
* Parallax Software
* Planet Moon Studios
* Quicksilver Software
* Rage Software
* Reflexive Entertainment
* Shiny Entertainment
* Sir-Tech Canada
* Treyarch
* VIS Entertainment
* Volition
* Xatrix Entertainment
:)

What's unusual about this list ? I don't get it :)

Bioware is.
EDIT: Oh, wait, it's MDK2. For a second I started thinking Baldur's Gate and getting all giddy. Never mind, then.

Heh, it would be unusual if it had some empty categories, then we would know what to expect.

Right now the only two developers is Interplay and Codemasters. I suspect GOG put those there because they're getting games from them. :)

Unfortunately you are not right. Interplay and Codemasters are the only two publishers. You quoted just the list of developers which games were published by this 2 companies. Try to filter the cataloguee by the publishers and you will see that each of them has at least one game here at GOG.