[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.