The great thing about the GTR 20 (or any remote LRU) is it can be installed anywhere in the airframe. I usually take into account where the antenna will be mounted and what kind of wire runs would be required based on distance. Sometimes it's better to run signal and power wires a long way to the LRU and then have a short run to the antenna, as opposed to putting everything up front and running a long run of coax with it's weight and signal attenuation. So sometimes my remote LRUs end up behind the panel, under the floor or aft of the baggage area based on the configuration and airframe.
Just make sure the LRU as a service loop and can be accessed easily for future maintenance.
(a few more airplanes too)