Title.

I realize this is a few months late...

Anyway, odds are that it will not work.

Reason:
GoG provides stand alone or Galaxy install.
Steam requires you enter a code or purchase through them (either way, it is through them).
As such, unless you can get a code from GoG then it isn't going to work.

Maybe it is possible, but I don't know of any way to get the key code for doing it.