GPS time unexpected isot representation

Description

GPS time is correctly defined as 1980-01-06T00:00:19 TAI as its epoch. Many text formats using GPS time, such as RINEX messages, have defined the ISO or ISO-like text datetimes for GPS to be TAI - 19s (leap seconds at the 1980-01-06T00:00:00 UTC epoch). With GPS time being specified via the format argument and not having a GPS scale since it follows TAI, the isot representation is not as expected by these standard formats.

Expected behavior

In [1]: t = Time(0, format='gps')

In [2]: t.isot
Out[2]: '1980-01-06T00:00:00.000'

In [3]: t.tai.isot
Out[3]: '1980-01-06T00:00:19.000'

As below under actual behavior, I would not expect the t.isot and t.tai.isot values to be the same. I’m not sure if this is a similar issue to #11245 or not, but seems like it could be. Not sure if this would need effectively a GPS time scale in addition to the current TAI to make it work. For now, the only work around I’ve found is explicitly removing the 19 leap second offset for the .isot in a wrapper method. Am I missing something key or is this the only way to handle this at the moment, and should this functionality be added? Obviously using t.utc.isot is not the correct answer as that would add all of the leap seconds since the GPS epoch for current datetimes.

In [1]: (Time(0, format='gps') + TimeDelta(-19, format='sec')).isot
Out[1]: '1980-01-06T00:00:00.000'

Actual behavior

In [85]: t = Time(0, format='gps')

In [86]: t.isot
Out[86]: '1980-01-06T00:00:19.000'

In [87]: t.tai.isot
Out[87]: '1980-01-06T00:00:19.000'

In [88]: t.utc.isot
Out[88]: '1980-01-06T00:00:00.000'

In [89]: t.gps
Out[89]: 0.0

In [90]: t.utc
Out[90]: <Time object: scale='utc' format='gps' value=0.0>

System Details

Linux-4.19.0-13-amd64-x86_64-with-debian-10.7 Python 3.7.3 (default, Jul 25 2020, 13:03:44) [GCC 8.3.0] Numpy 1.16.2 astropy 4.0.1.post1 Scipy 1.5.2 Matplotlib 3.0.2

[taldcroft]

See #11269.

[taldcroft]

I have a proof-of-concept fix which adds a new time scale gps_scale and gives the expected behavior in limited testing.

In [24]: t = Time('1980-01-06', scale='gps_scale')

In [25]: t.isot
Out[25]: '1980-01-06T00:00:00.000'

In [26]: t.gps  # SI seconds since the GPS epoch time 1980-01-06 00:00:00 UTC
Out[26]: 0.0

In [27]: t.tai.isot
Out[27]: '1980-01-06T00:00:19.000'

In [28]: t = Time('2020-01-01', scale='gps_scale')

In [29]: t.isot
Out[29]: '2020-01-01T00:00:00.000'

In [30]: t.tai.isot
Out[30]: '2020-01-01T00:00:19.000'

In [31]: t.gps
Out[31]: 1261872000.0