GPS tracks TAI, because TAI is an Atomic Time, it doesn't care about how the Earth moves in space. Some of the metadata transmitted alongside the GPS signal tells a device how to turn GPS time into UTC at the current moment in case you want to display that, but it's orthogonal to the main purpose of GPS which is to figure out where you are, for which you want a nice steady time system like TAI, not a civil system like UTC.

The way UTC works is, moment by moment it goes at the same speed as TAI, but every six months it might jump back or forward one second, to try to keep it roughly lined up with UT1. This is a compromise to give TAI's simplicity and reliability (atomic clocks are easy) but without gradually shifting away from UT1 over decades.

Why? Well, UT1 reflects the thing humans think "days" are about, the uneven wobbly spin of Earth, the big round thing we're almost all stuck to. If the Earth spins a little more slowly or quickly than expected, that's reflected in UT1 but doesn't affect TAI at all.

So in practice, you have to rely on what national labs provide. They provide time that is close to UTC but is not actually UTC, in the case of GPS they use the approximation of UTC by the US Naval Laboratories.

Now because UTC is the official civil time, and it is easy enough to compute TAI from UTC, what such labs provide is UTC.

Then internally, GPS translates that to GPS time. Which differs from TAI by 19 seconds.