Each device can report song and preset changes to the other using bank and PC messages. The GT can send CCs to the PBC to control the PBCs loops. All these things work like a standard MIDI device does, and all of this is independent of how your buttons are assigned. There isn't any direct mapping of "this button on this controller turns on that button on that controller". It's more like "this button selects the third preset in the fourth song, which tells the other controller to select the third preset in the fourth song". Or, "this button sends CC#30, which turns on loop 1 on the PBC".