Since the G5n doesn't do MIDI, you're SOL on using both at once.
Bottom line - HW modelers are best for LIVE playing - no latency.
You can use the FCB to control the presets on up to 5 instances of up to five plug-ins on up to five separate tracks at once.
But it sucks for LIVE playing because of the latency of plug-ins.
Modern modelers like Helix can do lots more than your G5n, including most of the MIDI capabilities of BOTH the FCB and plug-ins, making you forget all about plug-ins like GR5 except for post-processing/re-amping, but are pricey.
Again, make a list in priority order of what you NEED from your rig vs what you'd LIKE from your rig.
Some people feel that lists are old-fashioned and a waste of time, but they can be very helpful in situations like yours.