If on starting the game you find the gun in your Left hand please try to "start the game with your controllers off and then turning the left one on, followed by the right one." as suggested by Valve for their new update.
http://steamcommunity.com/games/719950/announcements/detail/1648749673594791163
"Removed the temporary workaround to reverse controllers with the right thumbstick: the right thumbstick will now open the SteamVR Dashboard just as the left thumbstick does. If you still encounter a game that reverses your hands, try starting the game with your controllers off and then turning the left one on, followed by the right one."