If I do opt for the Steam OS will my GOG Galaxy run on it or will I just have to launch the games directly? Most of the titles I play (other than the Halo games and Forbidden West) are using the Galaxy app.

What GOG support will there be from SteamOS, if any?

You’ll be able to access those games through the Heroic Launcher, it should be able to handle any GOG games you have just as easily as Steam will handle Steam games.