GMT is a time zone officially used in some European and African countries. The time can be displayed using both the 24-hour format (0 - 24) or the 12-hour format (1 - 12 am/pm).
UTC is not a time zone, but a time standard that is the basis for civil time and time zones worldwide. This means that no country or territory officially uses UTC as a local time.
UTC has leap seconds to keep it aligned with earth's rotation. Otherwise all timezones would slowly shift away from having any correlation with solar time. Between UTC and IAT, UTC is the more human-useable and thus better.
Ive been using utc personally for over a year and i use it in context of vrchat since it yields one less necessary conversion to other people's timezones because only the offset is needed (as opposed to memorizing both offsets, which is much harder because of that nasty nasty daylight savings and its weird anomalies) but they still hate it and tell me to use a "normal" timezone lol. I had gotten 1 person to switch. And she since switched back. Shit don't work in practicality but I'm still gonna use it out of stubbornness
Go play EVE Online. The servers used to have (still, do I think, but shorter) daily downtime that was scheduled using UTC and it led to everyone using UTC since the game server itself used that time.