The GMT function uses the same system as the cannon pinion, minute wheel, hour wheel. The hour wheel meshes with a pinon on the underside of the cover connected to a wheel above the cover that drives the GMT wheel. As the GMT turns every 24 hours, the ratio between it and the hour wheel turning every 12 hours is 2:1.
The more complex element is the GMT time setting (which comes in addition to the normal time, winding and date). This is achieved with the date quick-set wheel serving a dual function and moving to mesh with different pinions depending on the direction the crown is being turned. This way, the keyless works can achieve 4 functions with only 3 crown positions.
