We are using the GTR-200 in a J3, a Stearman, a 180 Super Cub, and this week one arrives for the J4. These are all aircraft with more or less demanding intercom environments, and we are using the internal intercom feature. It is a "stand alone" unit.
Dirt-simple. I shield all headset wires and use a return wire, with isolated jacks. That is only four shielded pairs, with shields terminated on the radio plug jacket. Four PTT wires, power & ground, and an antenna wire. I showed the Great Lakes guys how to do it, and they were successful on the first attempt. In the Cubs we never use the intercom ptts. Some of the Stearman guys use them, but open mic works for me.
The only downsides of the 200 is its limited RF squelch range (very susceptible to noise; we can clearly hear approach control on the tower frequency, and the car dealership at the end of the runway is actually painful) and its desire for a good solid 12 volts. Battery only we have to use 24 volts. Other than that, it is the best aircraft radio I have ever used.
I am going to yank a perfectly good SL-40 out of my personal Cub and replace it and its external intercom with a GTR-200.