Greenwich Time vs UTC

Greenwich Time and UTC both are nouns.

Word	Noun	Adjective	Verb	Adverb
Greenwich Time	Yes	No	No	No
UTC	Yes	No	No	No

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

Greenwich Time: 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 Greenwich Time include coordinated universal time.

Greenwich Time (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