[RE-wrenches] GFCI generator outlet

[All Solar, Inc.]

Wrenches 
I recently visited a site where their backup gen has GFCI outlets used for input to the FX inverter. 
They finally tried to charge the batteries, and it trips the GFCI. 
As a test I replaced it with a standard outlet and it works fine.  
I have never used a gen with GFCIs, In fact most are hard wired.  
Is the outlet the problem or is there really a ground fault?
Is there a safety issue?
I'm pretty sure the neutral and ground are bonded at the mains panel. 

Thanks 
Jeremy Rodriguez
All Solar

Sent by Jeremy's iPhone. Sorry for typos and shorthand!