What is the Full Form of GMT ?

GMT - Greenwich Mean Time

Greenwich Mean Time is used as a benchmark from where all time zones are measured. It was established in 1884 at the International Meridian Conference, when it was decided to place the Prime Meridian at Greenwich. Greenwich is a borough of London, England, located on the southern bank of the Thames River.

GMT is sometimes called Greenwich Meridian Time because it is measured from the Greenwich Meridian Line at the Royal Observatory in Greenwich. It is the mean (average) time that the earth takes to rotate from noon-to-noon. 

What is the Longitude and Latitude of Greenwich, England?
Longitude 0° 0' 0"
Latitude 51° 28' 38"N (North of the Equator)

Greenwich Mean Time is an absolute time reference that doesn't change with the seasons.