Short Question: Does someone know, how to use or even get the alienware skin for the gog version? Or is this steam exclusive?

The codes given out at the events were for the steam redeeming system. The devs mentioned making them available for all users at a later point, but right now we're out of luck here.

Ah, thanks now im informed. Would be a shame if we couldnt use the great alienware skin sooner or later.

What do these skins even look like?

One was a blue patterned version of the female coop-ninja skin (basically the same model as the Kyokagami twins, but with a mask). The other was a red-patterned version of the male coop-ninja skin with the gas mask.

Both were just colour variants of the base model, so you're not missing much.