GMT is dead
Greenwich Mean Time, or GMT, is mean solar time at the observatory at Greenwich. “GMT” is also often used, especially by British media, to refer to winter civil time in the UK, or to indicate times for events outside Britain. I’ve even seen it be used as shorthand for UK civil time in the summer.
“GMT” is a natural choice because mean solar time at Greenwich was the basis for civil time for a majority of the history of widely coordinated timekeeping. GMT is still perfectly well defined, but it is no longer the basis for civil time. Instead, civil time in almost every country, whether by law or in practice, is based on Coordinated Universal Time (UTC), a time standard based on atomic clocks. If you think that’s too long, just say “Universal Time”, that’s close enough.
What’s worse, GMT is no longer precisely measured. Nobody can tell you what the time is in GMT right now with any precision, unless you happen to operate an astronomical observatory, while millions of clocks around the world, including Internet connected computers, GPS receivers and “atomic clock” wristwatches, can tell UTC.
Conclusion: Don’t say “GMT”. It makes you sound old, unscientific and imperialist. Say UTC, Universal Time, Coordinated Universal Time, or Zulu time.