GMT is now ambiguous and there's nothing official that says what it means. In science, UTC has taken over as the main time standard, based on atomic clocks. The old definition of time used by GMT has been superseded and it's no longer clear what it might mean, though any of several standards based on the rotation of the earth, like UT1 and UT2, could be used.

The name GMT is still used by the British public to refer to the civil time used when summer time is not in effect. But there's no law or regulation that gives a definition of GMT or even how civil time is defined in the UK. The British body responsible for civil time has taken the obvious course and uses UTC.