"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).
Bulgari Octo Finissimo Chronograph GMT
Dubois Dépraz Retrograde Seconds, GMT & 24 Hour Indicator module. Reference 15115
Grand Seiko Hi-Beat 36000 GMT SBGJ219
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)
Breguet Classique Hora Mundi 5717. Functions: Central seconds, minutes and hours. Instant-jumping time-zone display with synchronized date, day/night indication and city.