Generally it’s not a very good idea to use something like a Y-cable to combine two signals! Those cables are used to split a signal from one source to two targets, and even then it’s probably not the best way to do this. In short: forget this kind of cable!
The cheapest way to go (and it’s a safe way too!) is to combine the L/R channels of the stereo signal within Gig Performer and then simply use only one output of your audio interface to go into the theatre’s mixer.
Theoretically you could just connect both audio pins (L/R) of a stereo sum with one audio pin of your audio interfaces outputs (this would be the digital version of your Y-cable, but there would be no hassle with electrical issues like impedance and all this stuff):
But to be on the absolute safe side, and if you are probably concerned about possible phasing issues, you might even use one of those free stereo-to-mono plugins… i.e. this one:
Just connect the plugin/audio sources from a stereo sum to this plugin and then just activate “Side Mute”, which will keep only the center component of the signal and make a perfect mono signal… this should do!
The wiring in GP would look like this:
