So I preordered Retro City Rampage directly through the developer's website. I read on the dev's website that I would receive either a Steam or GOG key for the game. After I purchased it, I did receive an e-mail receipt and the preorder bonus package but not a key. I was expecting to receive my key via e-mail right away (kind of like how Steam lets one preorder a game and it shows up in one's library as a preorder).

Does anyone here know if I will receive a GOG key for the game once it's released?

If that's what the developer's promised, then I'm sure you'll receive it.