Hi,
got you ... guage refers to wire size, for your application you will probably need either 14 guage or 12 guage stranded conductor, green outer insulated wire, suitable for use out of doors. your local hardware type store should be able to provide the details. 14 is suitable for a 20 amp, 120volt circuit and 12 guage would be needed for a 30 amp, 120 volt circuit.
in lew of a ground rod, you could connect the grounding lug, via this wire to the frame of the trialer, this would be a bonding path from source to load. this would fool the Good Governer into thinking everthing was correct.
a less practical, but probably more code correct arrangement by NEC 2002 code would be to attach a grounding wire of correct guage to an earth ground staff, describe in the code AND to install a second ground wire from the metal frame of the trailer to a second ground staff, describe in the code, ensuring proper sizing of rod and driven to correct depth ... in order to fool your Good Governor.
But then you would have to move that mess evertime you moved.
If you have a simple three light phase, polarity checker that can be plugged into a standard 120volt outlet to check for proper wiring and you plug it into the generator and it shows correct, phase and polarity, you are probably ok.
UL is pretty picky about that kind of stuff before the approve a generator for general use by the public.
happy trials, grey ghost