I have been using Garmin GPS device for years. Just curious which algorithm is used in the device:
Navigation equations that can calculate both GPS device's position and its clock bias. In this way, the time display should be accurate with the clock in the satellite. However, the computational complexity could be very high. See https://en.wikipedia.org/wiki/Global_Positioning_System.
Use TDOA method similar to what is explained at https://en.wikipedia.org/wiki/Multilateration. The computational complexity could be much lower, but you will not be able to obtain the receiver's time bias.
Note: This question is based on the assumption that the satellites' positions and TOFs have already been extracted from the received satellite signals.