I know that Desura and GamersGate offer Steam codes for this game if you bought it on their sites.
Will GoG be offering a Steam code?

GOG doesn't offer Steam keys. Presumably, some game developers can provide them if asked, but GOG itself doesn't do this.

He might be asking because the game just released on Steam and people are trying to get Steam keys for their previous purchases of this game. My Groupees Bundle of the Dead seems to have gotten one, no key yet for my GG Indiefort bundle though but perhaps later.

It's possible though that people who bought it here and want it may have to go through the dev itself and try their luck.

Just buy it directly from the devs.
They provide a Steam key along with a regular download.

I'd already bought the game on GoG.

But I saw it is now on Steam, that's why I was wondering if it would be possible to get a Steam code through GoG.
I just like having all my games in one place. Makes it convenient.

I'm not sure why you'd want a Steam key... You got a GOG key; which is way better! :)

Seriously though, I can't think of anything you can do at this point - except buy it again on Steam. If you could prove you owned the game through GOG somehow, the Devs might send you a Steam key... but I can't think of any way you could do that... Maybe take a screenshot of your GOG games page and send it to them? It might be worth a try at least - if you're desperate.

Since this thread was resurrected, I might as well throw my two cents here ;)

Steam client has an ability to display non-steam games on list. I do not know if this solution would satisfy you, but it can be a step closer to having all your games in one place.

Here is a link to Steam Support Page with some more details:
Add Non-Steam Games to a Library

HTH