Wasn't there a bunch of stuff for pre-order? Also nice to see GoG give away another game for late bloomers :P
oh, forgot to mention, having great fun with the game.

Just the soundtrack and a mercenary class to choose at the start in character creation.

Mercenary DLC: Do you see Mercenary class during character creation? If yes, you have it installed. If no, try to update your install or download manually.

The soundtrack can be downloaded:

If using GOG Galaxy 2.0:
Library > Biomutant > Extras and click download on the thing you want to download.

If using the website:
Your user name > Games > Biomutant and you will see all offline installers and extras.

edit: Oh yeah, I found that Shadow Warrior 2 on the games list on the website, but not when using GOG Galaxy 2.0. On the GOG website, go to Your user name > Games > Search > shadow
A bit weird that I can only see it this way, but hey, it's a free game.