We are using the GTR-200 in open cockpits (Cubs, Stearman, Great Lakes) and using its internal intercom. Even in the Stearman we can talk without changing squelch with power settings. Best open cockpit intercom I have yet used.
I assume the 225 is the same; just TSO?
I installed intercom PTT switches, but will not do so on my personal Cub, since I never use them. A couple Stearman partners have inferior headsets, so they go to 100% squelch and use the buttons to talk.
There are two downsides to the GTR - one is that it is power hungry. It sucks a 12 volt battery dry in half the time my SL-40+ PM 501 + Garmin 496 does. Batteries go back and forth.
And the other downside is the RF squelch circuitry. We have them all set at max, and local emitters drive us nuts! That won't be true if you operate in the country - ours pick up used car dealership intercoms and simultaneous transmissions on two ATC frequencies with difference equal to our north tower frequency. Garmin says they are looking into it, but so far nada.
Still, it is the best comm I have ever used; its display and memory circuit is light years ahead of the SL-40, and again - the best open cockpit intercom I have ever enjoyed. Installation is trivial if you know how to connect shields and have a crimper.