GFCI outlet reading between 80 and 640 volts on multimeter
One set of terminals says line that is the incoming hot.
The other set of terminals that normally has a cover over them say load.
The load is for additional receptacles to be protected.
Some brands the line is on the bottom some line is on top so we don’t know if it is correct or incorrectly wired.
Strange readings on your meter are you sure it’s on Volts and not millivolt range.
Just wiring wrong cannot create higher voltage.