Atomic clock timing services that existed before GPS didn't use leap seconds either. GPS time is related to TAI time, which is based on absolute elapsed time so

Atomic clock timing services that existed before GPS didn't use leap seconds either. GPS time is related to TAI time, which is based on absolute elapsed time so it slowly drifts away from UTC when leap seconds get added. IIRC, GPS time is just TAI/TT time with a different epoch.