So I bought TW2 for 60% off during the Steam sale. And I just noticed you could get the DRM-Free version from GOG if you registered your cd-key(which I did). So I was wondering if I could replace the steam exe with the gog one (and where I would get one), short of having to download the whole 18gb frrom GOG again?

I think that might actually work. When patch 2.0 and 2.1 had a bug that froze up the launcher when you started it up offline, the only recourse was to start the game from the .exe, something that Steam users were unable to do. Only after getting a GOG .exe and replacing the two were they able to make it work offline.

On the other hand, that was out of necessity and we're talking about an older version. It may not even work anymore; using a 2.0 .exe on a 2.1 version didn't work, and a lot has changed since then. Honestly, I'm not willing to provide an .exe because the legality behind it isn't really clear. It's unlikely that you'll be able find it anywhere out there on the internet, either (you'll most likely end up with an outdated version that doesn't work).

If I were you, I'd just bite the bullet and download it from here. If you're intent on trying this, though, you should go to the official forums and ask the developers about the legality of it. If they say it's fine, I'd be more than happy to upload my .exe for you.

You have no guarantee that the only modified file between GOG and Steam distribution is the exe, so you can end up with a messed package that a) might not even start, b) might start but behave erratically c) might work well but will be screwed in a future patch...

This is just asking for trouble, IMO... if you have already downloaded the Steam version, go ahead and play (the only DRM is Steam itself anyway), if you want the DRM-free one, uninstall a do a clean download+install from GOG