UTC only has leap seconds to keep within 0.9s of UT1, which is the mean solar time at 0° longitude (based on Quasar measurements). GMT is essentially equivalent to UT1 (and in modern usage either means UTC or UT1).
UT1 essentially doesn't need leap seconds, because it is the correct mean solar time and not an 1Hz approximation of it (which UTC pretty much is).
GMT does not observe leap seconds for instance.