I don't know why Z was used instead of UTC or GMT. Probably to save space, and save some ink if a schedule was printed.
ZULU time is military, primarily NATO. The world is divided up
into alphabetic time zones using the NATO phonetic alphabet with a
few exceptions like India (the letter)
Being extremely pedantic, GMT and UT and UTC are all slightly
different, UT and UTC particularly. GMT is now tied to UT. UTC is
more precise than UT/GMT