Yes, for some reason (I imagine to force people to use Galaxy) GOG hide the game exe file.
It's on bin\x64 in your installation directory.
That's rubbish. GOG don't hide anything. The game was not made by GOG, it was made by CD Projekt Red, and the game executable resides in that folder by design. People who have Galaxy are assumed to want to run the game through Galaxy, that's why there's a hook to run Galaxy.