GMT is a time system referring to the mean solar time at the Royal Observatory in Greenwich, London. Later on it became an international time standard. GMT and Universal Standard Time (UTC) can be seen as the same as long as fractions of a second are not important.
Noon Greenwich Mean Time is rarely the moment when the sun is at the Greenwich meridian, meaning the moment when the sun is at its highest point in the sky at Greenwich. During a year the difference is up to 16 minutes. Therefore, the word 'mean' was added to the Greenwich Mean Time, meaning the average mean time during a year when the sun is at Greenwich meridian.
Historically there has been two conventions to understand Greenwich Mean Time, the zero moment at noon or at midnight. However, with Coordinated Universal Time (UTC) it is always referring to midnight as zero hours.
The world followed Greenwich Mean Time till the 1960s when the atomic clocks took some of the responsibility. However, the atomic clocks cannot be fully trusted since after a while there is dysynchrony between the atomic clocks and the rotation of the Earth. The atomic clocks are very accurate but the rotation of the Earth is constantly slowing down. This will create differences between the atomic and the natural time.
The aim was to have a time which is synchronized with the rotation of the earth. A new time called Coordinated Universal Time (UTC) was created. There was a dispute between the French and the English of the shortcut of the new time name. The French suggested a short cut TUC, Temps Universel Cordonné, and the English CUT from Coordinated Universal Time. As a compromise, UTC was chosen.
The accuracy of UTC is as good as the accuracy of atomic clocks, but it is adjusted when needed to keep it synchronized with the rotation of the Earth. In the beginning of the 1960s UTC was only used in research centers in the United States, in England and in a few other countries. For example in 1964 UTC was adjusted in the beginning of the April and September for 0,1 seconds and again in October for 0,01 seconds.
The continuous adjustments were troublesome, therefore in the beginning of the 1970s the definition of UTC was altered so that the adjustments are done when the difference of atomic time increases close to the next full second. With this modification it was possible for UTC to be used internationally. In the beginning of the 1972 the new UTC was adopted globally.