Hey Andrew, I just went through this same question process myself, so I'll try and answer here. Basically, the GTX23ES is an "ADSB Out" transponder (no "in"), and connects to its own transponder antenna. Unfortunately, it doesn't do "ADSB In" to get things like traffic and weather information though. In order to get that capability, you need ANOTHER box, the GDL39R, which connects to a separate transponder antenna. Hence a lot of folks these days, including probably me, need two transponder antennas.
Garmin does make an all-in-one box called the GDL88, but it's a lot more expensive (about $1000 more than the other two boxes combined, if I remember).