In one embodiment of the present invention, there is provided a gas gauge for use in a vacuum environment having a measurement gas flow channel. The gas gauge may comprise a measurement nozzle in the measurement gas flow channel. The measurement nozzle may be configured to operate at a sonically choked flow condition of a volumetric flow being sourced from a gas supply coupled to the measurement gas flow channel. The gas gauge may further comprise a pressure sensor operatively coupled to the measurement gas flow channel downstream from the sonically choked flow condition of the volumetric flow to measure a differential pressure of the volumetric flow for providing an indication of a gap between a distal end of the measurement nozzle and a target surface proximal thereto.