No Outlets on generator

I would guess the generator worked properly in the past. You'll need to check your generator wiring. Either the neutral or ground wire may be loose or disconnected causing an imbalance which trips the GFI. On shore power the house ground and neutral are bonded so the GFI works properly and doesn't trip.
Some generators have a floating neutral. There is a special plug made and sold that plugs into a generator outlet that bonds the neutral and ground. If you can't figure it out please take to a dealer so you don't get zapped.