I was initially confused by the `= 00:00:19 TAI` part, so I would suggest removing that.  The date of `1980-01-06 00:00:00 UTC` is unique and corresponds to the definition that one sees everywhere.

It might be useful in the class docstring to include a reference to some fundamental page that defines GPS time, e.g. http://tycho.usno.navy.mil/gpstt.html.  You should have the initial 'tagline' of `GPS time: seconds ...` and skip a line and then have more detailed explanation, which might include mentioning that this does include leap seconds.