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Regarding your first point: Is GPS time defined for dates before 1980? If not it does not have leap seconds in the sense that it skips or inserts seconds at specific times. It just has a constant offset to TAI, I guess to maximise the number of leap seconds that GPS can handle in its 1 byte leap second field.

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http://www.usno.navy.mil/USNO/time/gps/usno-gps-time-transfe... says: "The GPS epoch is 0000 UT (midnight) on January 6, 1980."

So no, gps time is not defined before 1980.

So, in the time period GPS is defined it contains no leap seconds.

If that is the case why is it not the same as TAI?

The GPS signal carries information about the GPS vs. UTC drift in a 1 byte field. As said, my guess is that they just wanted to maximise the use of this field by setting it to 0 instead of 19 at the beginning.

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