The GMT hand is a fourth hand that completes one full rotation in 24 hours instead of 12, allowing you to read two time zones simultaneously on the same dial. This is the fundamental distinction that separates a GMT watch from a simple bicompax: it's not enough to have a 24-hour scale engraved somewhere—you need a dedicated hand that travels across it.
The contrasting color—red, orange, yellow, blue—isn't purely aesthetic. It serves to distinguish it at a glance from the hour and minute hands, especially in low-light conditions. For the same reason, many GMT hands are treated with Super-LumiNova: the substance absorbs light and releases it in the dark, making the second time zone readable at night. The shape of the tip varies: full arrow, triangle, lollipop (round disc at the apex), cut-off tip. These are stylistic differences, not functional ones, though full arrows and triangles tend to be more legible from a distance.
From a mechanical standpoint, there are two distinct families. In the so-called office GMT—or traditional GMT—the additional hand is integral to the gear train and cannot be adjusted independently: you set the local time, and the GMT hand follows. This is the most common solution in mid-range calibers, reliable and less expensive. In the flyer or true GMT, however, the local hour hand jumps forward or backward in one-hour increments via the crown, while the GMT hand continues to rotate without interruption: the reference time zone is never disturbed. This is the preferred configuration for those who work shifts or travel frequently, because the home time zone remains always stable. Some manufacturers add a further variant: the independent GMT, where the GMT hand is adjusted separately from the local time, with significantly higher production costs.
A note from the bench: on the Rolex 3185 caliber of the Explorer II, you may observe a slight oscillation of the GMT hand during adjustment. It does not compromise reading accuracy, but those unfamiliar with it worry. It's a constructive characteristic of that caliber, not a defect.
The 24-hour bezel that accompanies many GMTs does not replace the hand: it complements it, allowing you to catch a third time zone by rotating the bezel. They are two tools that work together, not as alternatives.