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.
maybe we drop one vestigial nod to the middle ages and compromise: BC/YL
I have a bit of an issue with it because it does seem to endorse a religion, but still.
In fact CE makes that origin the 'common' default.
It’s old enough that it skips from 1 BC to 1 AD.
And I'm steadfastly among the ranks of grumpy old farts who still use AD and BC.