This paper describes an efficient algorithm, based on the Gradient method, for the fine estimate of the Doppler shift of Global Navigation Satellite System (GNSS) signals. The algorithm is a suitable solution for software radio implementations, as it requires a moderate computational burden. The new approach is based on a close-form expression that gives an effective way to refine the estimate of Doppler shift without using conventional Frequency Lock Loop (FLL) architectures. The algorithm has been assessed in simulation and tested with live GPS signals. Compared with the classical FLL-based architectures, a shorter transient time between the signal acquisition and the tracking phase is achieved.