Thinking it'd be a good idea to open 1-2 Arma 3 servers and see how they go, specifically Altis Life and Wasteland.

I also think that opening a few more Gmod servers wouldn't be a bad idea? I haven't been on personally but looking at the numbers on TTT it seems to be doing well.
Maybe explore Murder, DarkRP? Some other mods? I don't play Gmod all that often so i cannot suggest to much.

Obviously to get these servers running well you need a fairly solid backing, people who are willing to jump on and play even if there aren't all that many players online at the time. Nobody wants to join a server with 0 players on it so you can't just expect regulars when its stuck at 0 players all the time.

Gmod i feel wouldn't have this problem because there is already from what i can see a solid amount of people on the forums who contribute to the Gmod section and probably wouldn't mind playing the other game modes a bit, as for Arma 3 i'm unsure. It could just fill up itself after a little push. Most people who play Arma 2 or Arma 3 find a server and then stick to it so getting regulars isn't a really annoying task.