GMT vs UTC

GMT and UTC both are nouns.

Word	Noun	Adjective	Verb	Adverb
GMT	Yes	No	No	No
UTC	Yes	No	No	No

As nouns, UTC is a hyponym of GMT; that is, UTC is a word with a more specific, narrower meaning than GMT:

GMT: the local time at the 0 meridian passing through Greenwich, England; it is the same everywhere
UTC: Greenwich Mean Time updated with leap seconds

Other hyponyms of GMT include coordinated universal time.

GMT (noun)	UTC (noun)
the local time at the 0 meridian passing through Greenwich, England; it is the same everywhere	Greenwich Mean Time updated with leap seconds