The dependence of the piezo-optical strain sensor gauge factor on the geometric parameters of the photoelastic element is obtained by accurate numerical simulations. It is shown that the piezo-optical gauge sensitivity to the applied force is more dependent on the photoelastic element shape than the sensitivity to deformation. A comparative analysis of the gauge factors and other parameters of strain-resistive, piezoelectric, fiber-optic, and piezo-optical strain gauges was carried out. Strain-resistive and fiber-optic sensor gauge factors are three orders of magnitude lower than those of piezo-optical sensors. Correctly calculated piezoelectric sensor gauge factors are also two-three orders of magnitude inferior to those of piezo-optical sensors. A comparison of the basic properties and parameters of modern commerical strain gauges based on different physical principles is presented.