A GMT and a dual time watch both show you a second time zone, but they do it differently. A GMT uses an extra 24-hour hand read against a bezel or marked dial, while a dual time uses a separate sub-dial or a second movement to display the other zone. The GMT is the more capable travel tool.
The short answer
If you see a fourth hand that makes one full rotation every 24 hours, you're looking at a GMT. If you see a small separate dial showing a different hour and minute, that's a dual time. Both tell you what time it is somewhere else, and a GMT can often track a third zone using its bezel.
How a GMT works
A GMT watch adds a 24-hour hand that circles the dial once a day. You read it against 24-hour markings, usually on a rotating bezel. Because the hand covers 24 hours in one rotation, it can't be confused with AM or PM. Rotate the bezel and you can even track a third time zone.
How dual time works
A dual time watch shows the second zone on its own sub-dial, often as a smaller 12-hour display. Some use a completely separate movement inside the case. It's simple to read at a glance because it looks like a second little clock on your wrist.
Which is more useful for travel?
Frequent travelers usually prefer a true GMT, especially a flyer GMT where the local hour hand jumps independently. That lets you land, set local time in seconds, and keep home time on the 24-hour hand. A dual time is perfectly good if you just want a fixed second zone you rarely change.
Reading AM vs PM
The GMT's 24-hour scale is its quiet advantage. It tells you whether it's day or night back home without guessing. A 12-hour dual time sub-dial can leave you unsure, though many add a day/night indicator to solve that.
Which should you choose?
Choose a GMT if you travel often or want to track multiple zones with one clean, capable complication. Choose a dual time if you like the look of a second sub-dial and only need to keep tabs on one other zone. Both are great, but the GMT is simply the more flexible tool.