The UNS is not required for use when using the GTN. There are some intricacies to be aware of (modeled after how the real avionics work). The UNS feeds the autopilot via GPS Steering, so it gives very nice smooth turns, with adequate prediction, etc. The NAV inputs to the EADI/EHSI system do not have GPSS, so the GPS from the GTN is treated just as a standard nav radio input, which means it may not fly as nicely as a full modern suite of avionics.