I was always taught that true altitude was indicated altitude, corrected for any altimeter error and then compensated for non-standard temperature, which should then be you actual altitude above local sea level.
Most GPS Altitudes are against the Geoid (probably EGM96 of Giod99) not WGS84 ellipsoid and as such are typically are pretty consistent with Altitude above local sea level and very consistent for altitude vs locally charted terrain on modern maps in Western countries (where the elevations will typically have been determined vs the Geoid).