Pedestrian navigation system based on foot-mounted inertial measurement units is considered. The problem of gyroscope biases estimation with the help of zero-velocity updates in case of simplified motion equations usage is studied. Analytical observability analysis done with several assumptions shows that gyroscope biases are observable. However numerical analysis done for simulated data indicates that “vertical” gyroscope bias estimates convergence is slow which is unacceptable for practical applications. Further studies should uncover the reason for such behavior of formally observable variables.