GMT, the famous Greenwich Mean Time, is used everywhere. However, we should all consider not using it. Universal Time (UT) — or even better, Coordinated Universal Time (UTC) — should be used instead. Why? In a given location, the real solar time is the time indicated by the Sun on a sundial. It is defined as the angle, expressed in hours, minutes and seconds of time — 1 hour corresponding to 15° — between the sun and the local meridian. According to the definition, it is then 0h when the sun crosses the local meridian, which is noon local time, and not 12h! The mean solar time is the real solar time corrected by the equation of time. The Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, London.
As you can see, GMT refers to an astronomical day starting at noon, and is thus not designed for civil timekeeping. Universal Time is the Greenwich Mean Time adjusted to have a civil day starting at midnight.
Universal Time (UT) is not a perfectly uniform timescale (the Earth’s rotation is not perfectly uniform) To compensate for this lack of accuracy, we now use Coordinated Universal Time (UTC), which is an atomic timescale (thus perfectly uniform) that approximates Universal Time (UT) by less than a second (remember hearing in the news that we need to add or remove a second to our clocks at the end of the year? Those are leap seconds to keep UTC in sync with the Earth’s rotation!)
Do yourself a favor and stop using GMT. Look smart and start using UTC instead. Cheers!