Vintage mechanical Timex watches with GMT functionality did so through the use of special bezels.

12 Hour bezel

The 12 hour bezel allowed the user to position the 12 o-clock position in the local time zone and read the time using the bezel instead of the markings on the dial.

World Time Bezel

The world time bezel features the names of cities throughout the world. Moving the bezel to position your chosen city at local time lets you tell the time in 2 different timezones.

Both of these GMT bezels are made from plastic, are friction fit and bi-directional. They are easily damaged and highlighly sought after by collectors in good condition.