Would I get a key trough the developer page after registering an gog.com copy? Just wondering if I can have GOG as an option to get it. I remember some dev doing this but not sure who >.<

Yep, once you register the GOG key on the dev website (creating an account there), you'll also get a Steam key.

Decided to activate the game on Steam now but can't find the "GOG key" anywhere. Could someone help a bit?

Did you buy it on Steam? If that's so, then I don't think you can get a DRM-free version.
If you bought it on GOG, then you can register your key (shown in your GOG library when you select Race the Sun) on the dev website, which will grant you a Steam key, too.