It’s just the politically correct way of saying GMT or Zulu, like you are supposed to say BCE and CE instead of BC and AD for years now.
And I'm steadfastly among the ranks of grumpy old farts who still use AD and BC.
It’s UT1 and UTC that differ by up to 0.9 seconds. GMT and UTC are equal unless you’re in a specialized field (navigation) where it is considered to be UT1. Otherwise, GMT is just the name of a time zone UTC+0000.
TAI measures seconds. UT1 measures the earth’s rotation. UTC combines these by using TAI plus a changing offset to keep within 0.9s of UT1.
I think that the bigger difference is that GMT is a time zone used in countries that observe daylight savings time. UTC applies universally, everywhere and always, but it's a little bit nonsensical to use GMT to describe events that happen during summer in the UK.
There's also a not-insignificant risk that the UK moves timezone at some point; if it stays in Europe it will likely be required to drop DST, and may end up on UTC+1. This would make GMT even more confusion-prone.