"Mean time at the meridian of the Royal Observatory in Greenwich, which is the prime meridian of the world. GMT is a civil time beginning at midnight." (FHH)
GMT is the time at the Royal Observatory in Greenwich, London. It remains the same all year round and is not affected by Summer Time (Daylight Saving Time) clock changes. Greenwich Mean Time is also a time zone, used by the United Kingdom of Great Britain (when daylight saving time is not in use).
Dual time (zone) watch
Describes a watch that provides the time in two time zones.
World time
"Describes a watch that indicates, usually by means of subdials surrounding the main dial, local time (true solar time) in different world cities. Often these cities were chosen for their political or economic importance prior to 1883 when universal time was introduced." (FHH)