What does G.M.T. stand for?
It is Greenwich Mean Time (G.M.T.), now more officially known as Universal Time (UT). This is the local civil time at the former site of the Royal Observatory in Greenwich, England, which is located on the prime meridian. It is used as the basis for standard time in most countries of the world.
This was decided at the 1884 International Meridian Convention in Washington D.C.
—The Fact Monster