[charriu]

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.

[protomyth]

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

[dfc]

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

[filmor]

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.