The ADS-B out data is fed from the aircraft’s ADIRU (air data inertial reference unit), which, yes, just uses GPS unless that fails for some reason. There’s a fallback based on accelerometers and laser gyros to provide shockingly accurate positioning should such a failure occur, but it does lose some accuracy (enough that you can still fly autopilot on it for an approach at max range, but it’ll be drifting by some number of meters) over long flights before it needs to be reset from GPS data on the ground.