After several updates, Gotham City Impostors is now completely playable on PC. Aside from the free content update, major changes include: they've overhauled the matchmaking system (got into a game in less than 2 seconds), fixed all the controller-focused UI to work better with a mouse, and there's now a FOV slider.
It's a shame that they didn't release this game playable. But at least they went back and fixed it back up. Community's still pretty active too (at least when I play). Just a heads up, folks!
(Note: game is still GFWL, unfortunately XD)
The game is a weekend deal on Steam as of right now. Go grab it here. It's 50% off at $7.49 USD. :)