Marc,
One idea I had is to build 3 Customs:
1st- The actual custom you want to trigger with the GPI but make the first command a Hold and the last command a loop
2nd- A CC that just resumes the first CC and attach the GPI to this one
3rd- A CC that Cancels the first CC (a blank CC also works but stops all CC's)
If you want to use the GPI trigger, hit the first CC, the GPI triggers the Resume CC and at the end loops and sits at the hold again for another resume command. If you don't want the GPI to work, cancel the the First CC. The resume should only work if the CC is at the "Held" state.
I haven't had a chance to try this so it is a theory at this point. Good luck.
#Carbonite