So, GPS involves a few things: precise clocks that can be used to triangulate position; a model of where the satellites containing the clocks are in space; and

So, GPS involves a few things: precise clocks that can be used to triangulate position; a model of where the satellites containing the clocks are in space; and a model of where the earth is in space. UTC has basically the same inputs (the clocks are on the surface instead of in orbit, but one of the main time labs [NIST] is in Colorado and its altitude needs to be taken into account), but the calculations and logistics, the timebase of the model, are all very different. So you can’t derive UTC from the GPS ephemeris, because there’s a human in the loop who decides when leap seconds happen. So GPS needs to include UTC as a separate signal, alongside the ephemeris and everything else. (In fact, if I remember correctly, GPS includes nanosecond-level corrections between GPS time and UTC, because atomic clocks are not completely perfect.)