Set the Midi out of GP6 to LoopMidi
Open RealGuitar Standalone set midi in to same LoopMidi channel
http://www.tobias-erichsen.de/software/loopmidi.html


You can use VSTHost to Load RG4.dll and Amplitube/Guitar Rig.dll and set VSTHost's Midi in to LoopMidi channel.
http://www.hermannseib.com/english/vsthost.htm

Or MiniHost Standalone and set MiniHost's Midi in to LoopMidi channel.
Join the FL Studio Forum and download free MiniHost Win or Mac
http://forum.image-line.com/viewtopic.php?t=123031

You can also put Kontakt after RG4 then Setup > Output Mode > Midi only
then put Amplitube/Guitar Rig after Kontakt. This way you can have Electric or Nylon using RealGuitar 4 Acoustic's Midi Out.


GP7 should have vst support and maybe have a vst version.