I have never actually posted in this topic, so I'm going to put my two cents in for anyone reading this.
I believe in adding what ever groups you want to Garry's Mod. Garry's Mod is built to be modified. You can build an entire game inside this game. What's stopping you from adding an Owner group? The answer is nothing. Almost everything can be modified to your liking, including an Owner group that is fully functional.
If anyone ever wants a nudge in the right direction, feel free to PM me.