Tripping gfi in garage when plugging into garage

The strange thing is that it was working for over a week. Had ac and microwave circuit breaker off to try and isolate. Even removed one of the leads to water water but still tripped. Water heater does not appear to be fried. It works on the generator with no problem. I was thinking the same thing that is why I tried it on the generator. Even tried a different outlet on the house which was not in the garage.