I think a TV mode would be really useful for the client. It would make it so much easier for us who prefer to play their games on a TV. Steam's "Big Picture" mode does a fine job by making the TV (or any secondary display that the user has defined as the big picture device) the primary display in the OS and putting it back to the settings before when exiting the big picture mode. It's a very convenient feature that I would love to see in GOG Galaxy.

I have added games to Steam as non-Steam games just to start them from big picture mode.