(Thanks to movac from Steam Gifts from the heads up.)

There was already a topic for this: Retro City Rampage on sale.
And of course this is why we have the Gaming Deals Thread.

Sorry, search on this forum is simply worthless. I did search.

Not a problem, the forum software is horrible (not just the search).

So I just bought this from the dev but humblebundle is only offering me a Steam key, not a GOG key. Does anybody know how I can get my GOG key?

Looks like from the official website (in "game features") that you can get either a Steam key or a GOG key. There are two different links.
I guess that if you got a Steam key you bought the wrong version...

It's a shame that it's not very clear. IMHO you should get both keys with the purchase.

If you didn't register the Steam key yet, try to contact the developer and try to exchange it for the GOG version.

I see that now. How STUPID!!! The white box where I purchased has no option to select one or the other. 99% of people will miss this and make the same mistake that I did. I e-mail humblebundle, let's see what they do. I can't find any place to contact the dev, but their website is hideous and so crowded with information that it is next to impossible to find what you are looking for.

Yeah, they made it very easy to overlook the GOG key option.

If you want to receive a GOG key, use this link to buy the game:

http://www.humblebundle.com/store/product/retrocityrampagegog

Yup. As a sidenote, Steam has been the default partner for the Humble Store/bundles, it's not really surprising that the Steam version gets the default sale applet. That's the first time I see a GOG key on a Humble Store game and it looks like it's sold like a different game. Yeah, the website is rather messy but there is a contact entry in the main menu. See there: http://www.retrocityrampage.com/contact.php