Transponder Mode C altimeter readouts are referenced to 1013 hPa. GPS altimeter readouts are referenced to mean sea level.
Yes. My point wasn't that a GPS altitude is in some weird way referenced to 1013, but that this is more or less the pressure at which the barometric altitude and the GPS altitude will be the same absent the offset that we enter in the Kollsman window to effectively bring them back into agreement with each other when reading off a barometric altimeter.