Okay, I looked up the GTX45R. Bottom line, you bought the wrong box to work with your GRT EFIS. A Trig TT22 plus a compatible adsb-in box (like the SkyRadar) will get you a mode-SES transponder (ES is the adsb-out part) and adsb-in, all of which can be displayed on the grt efis.The TT22 can be remote mounted and controlled by the grt efis. Both the 22 and the 45 require a gps position source. Both can work with a Garmin navigator (400w, 650, etc). The Trig will also accept data from grt’s $500 gps source.
PS. The Trig plus SkyRadar is cheaper, too.