In your circuit breaker box turn the breaker for the outlets "all" the way off then back on to reset.
A GFCI has a LOAD and LINE the "input" voltage is on the LINE side output is LOAD.
You need to measure between the black wire and the white wire then from the black to the green or bare wire on the LINE side to verify input voltage from the breaker box.
I apologize if you are familiar with electrical circuits and already know this a lot of folks don't.
MSW inverters do not have a clean sine wave output they have a "modified" sine wave and your meter is designed for measuring a clean sine wave a high reading especially under no load is normal.