On the developer website for $3.75 and includes a DRM-free version and a Steam Key (it says "or a GOG key" but I haven't figured that one out yet).

http://www.retrocityrampage.com/

You can choose a Steam key OR a GOG key.

Yeah, I figured that one after accidentally buying the Steam version. There's a link in the Game Features that goes to either the Steam version or the GOG version.

I wish the GOG one was more obvious and not hidden in small text below the purchase window.

Yeah, it's a shame :l

Yeah I figured it out too late as well.:/

You can pick GOG from this page too:

http://www.retrocityrampage.com/buy.php

and just keep an out for it under Game Features.

I contacted Humble support and, although they weren't able to switch my order, they did give me a refund so I could purchase the right one. Haven't heard back from the Retro City people yet, though, and I contacted them about an hour earlier.

Double bad luck. First I took the wrong version(steam) and then something went wrong. Payment went trough but I did't get any key. Hoping that support is gonna help me out.

Huh. What are the odds of it going bundle now?

That's a great deal, but from looking at screenshots and gameplay footage I can't judge whether I'd enjoy it and I've bought too many games that were a waste ... How is the gameplay like? Is it linear or sandbox? If I find out that I have difficulties with something or that I'm getting bored by it, can I just choose to do something else and different in the game? Can you save and reload? If so, when and how? Do you think players unfamiliar with most 80's and 90's console gaming might enjoy it, too?

The only thing I can tell you about it so far is that the file size is 25mb which with my current internet connection is a godsend!:D

They gave me a refund to so I could get the GOG version, great support service !

I assume the Humble Store version doesn't contain all the extras, or does it? So apart from account-based aspects, the choice between Steam and GOG is also a decision between achievements or bonus material?

Yep, the Humble Store key you get is only for the DRM-free download and either a Steam key or GOG key, depending on what you purchased. No soundtrack or other bonus content is available from the key/download page.

Already own it on Steam... picked it up again on GOG and DRM free though... bargain!