Not that I am going to do this, but couldn't this be abused in such horrible ways as you buy a game on Steam...connect to GoG and redeem your free copy. Then you ask Steam for a Refund?
What is stopping this from happening?

GOG reserves the right to remove the game from your GOG account if it's removed from your Steam library.
See question 15 here:
https://www.gog.com/connect

You could also skip all of that and just pirate every game on here. Apart from Bradley and a few others we simply choose not to do that.

Okay. That's good then!

Quoted for truth.

Yeah, but... DRM-free installer. As long as you downloaded the game from GOG, you'd still have it. Of course, if you're willing to go that route, you might as well simply pirate it (especially since I imagine that repeated abuse would get your GOG account closed), but if such an obvious exploit exists, I find it hard to believe that nobody will attempt to take advantage of it.

Mandatory remote activated explosive collars. That should solve it.

I'm good with this idea!

You can't just mindlessly refund every game on Steam. They have limitations.

There will be abuse, no doubt about that. You could apply the same logic, though, to the money-back guarantee, where someone pretends he has issues with the game to get it for free. Whatever is the case, there will be no access to patches (except for piracy) and no Galaxy-specific features (such as multiplayer).

Not to mention less risky as both sites could shut down their accounts for repeated abuse of service.

They will crack it in time.

Apropos of nothing, I love the feline dialogue upthread.

Refund should be the least worry.

Even I who already have Van Hellsing on Steam now can't use Steam refund, technically, as I'm past 2 weeks limit.

I could, however, by blind luck request refund and Lord Gaben grants it on a whim, but for how many times?