The only issue you may have is a connected generator may need to be more accurate on its AC frequency output.
If I recall correctly, GVFX has a narrower AC input frequency lock range. An inverter-generator should have no trouble. You might have to tweak the rpm governor speed to center it up on 60 Hz for 3600 rpm synchronous generator. Governor controlled synchronous generators need to have very stable rpm speed control. Nice and smooth sound. No 'wha-wha' sound pitch variation of a hunting, unstable rpm governor control.
Also make sure charging is not setup too high so it overloads generator or inverter will release shortly after connecting to generator.
Found old manual:
AC INPUT OPERATING VOLTAGE RANGE: 90 – 150 VAC
Voltages outside of this range may damage AC loads connected to the GFX’s AC output.
IMPORTANT: The GFX is designed to accept a utility grid quality AC input. Using a GFX with a generator as the AC input is
not recommended. Most generators, even high-power stable generators have been shown not to connect or charge through
the GFX. OutBack is working on this problem, but for the time being, do not expect your generator to work with the GFX.
'quality' in this context means absolute frequency and frequency stability. An inverter-generator would be best bet. They did not exist when this manual was printed.
