The acronym GMT stands for Greenwich Mean Time, but in watchmaking jargon it refers to a family of complications that allow you to read two time zones simultaneously on a single dial. The fundamental distinction is not between GMT and non-GMT: it's between GMT traveller and GMT office, and whoever doesn't know this difference buys the wrong tool.
In the GMT traveller, the hour hand is mechanically independent and moves in one-hour increments without stopping the minutes. When you land in Tokyo, you can update the local time without touching the second time zone. It's the system that makes a watch genuinely useful while traveling. The GMT office, also called caller or independent GMT, works the opposite way: the central hands remain on local time and only the 24-hour GMT hand moves. Perfect if you work with New York from Milan and never change time zones yourself; less practical if you're on a plane every week.
The historical progenitor is the Rolex GMT-Master from 1954, born for Pan Am pilots. Its GMT hand was mechanically coupled to the hour train: adjusting the hour wheel also modified the GMT hand. Today many call that system a first-generation GMT, because it wasn't yet a true traveller. The Dual Time function is even more basic: a second hand, often red, shows home time while you're away on assignment. No additional mechanism compared to a standard movement, just an extra wheel.
There are also alternative solutions to the central hand. The fixed bezel graduated in 24 hours, like on the Tudor Black Bay Pro, lets you read a second time zone by aligning the GMT hand to the scale engraved on the outer ring, without additional moving parts that could be accidentally shifted. The rotating bezel with city names — the so-called city bezel — is instead the most intuitive version: you rotate it to your reference city and read the local time directly. It's not a complication in the strict mechanical sense, but it works.
The GMT flyer is a less common variant: instead of a hand, it uses a rotating disc beneath the dial with 24 sectors. The Miyota 9075 adopts it with a colored indicator that moves along the perimeter. Less legible than a hand, but architecturally cleaner on certain dials. At the counter, I've had to explain this difference at least three times a week: whoever looks for a GMT often has the Rolex in mind, but doesn't know exactly how it works. The first question I always ask is: do you change time zones yourself, or are you watching a colleague on the other side of the world?