Several possibilities
- Connect a Relay to a MIDI Out (Local GP Port) in the Global rackspace, and then map incoming MIDI messages to widgets that can then control actions via a System Actions plugin - and be VERY careful to not use any MIDI In OMNI plugins or you will immediately get nasty feedback
- Alternatively use a global script (again, in the global rackspace) to receive those MIDI messages and then perform the desired operations by responding to the incoming messages in script callbacks
That said, since both Reaper and GP support OSC, it would probably be much easier to use OSC for these kinds of things