Greenwich Mean Time is the mean solar time measured at the Royal Observatory in Greenwich, London. For centuries it was the world's reference clock, which is why so many people still use it as shorthand for "the baseline".
Today its practical role is smaller: GMT is a time zone that certain countries observe, while UTC is the technical standard that defines all zones.
London is not on GMT all year
This trips people up constantly. The United Kingdom observes GMT in winter, then moves to British Summer Time (BST, one hour ahead of GMT) from late March to late October. So a meeting set for "10am GMT" in July is an hour away from what a Londoner will read on their phone.
If you mean London, say London. If you mean the standard, say UTC.