If you were tracking your own timezone (assume UK) and it's 12 midnight, all hands are aligned and pointing straight up.
On your watch face you have your customary 12 hour indicators represented one way or another but its represented as 12 hours and on any watch with these you'd know the current time. Whether it's AM or PM it doesn't matter because we'd know 9 in the morning vs 9 at night.
There's also a smaller set of numbers, either on the watch face or the bezel but it a smaller set. For sake of discussion I'll call it a chapter ring. The chapter ring is usually marked in a 24 hour increment. Some have all 24 numbers some have 12 (but spaced apart with a dot or other indicator in between) but its 24 and starts at zer0. You still following?
So, when it's midnight to everyone, GMT is 0 hours (oh-zero hundred)
Six hours later, hours and minutes and aligned vertically whereas the GMT hand is now pointing at 3 (but actually pointing at the 6 on the chapter ring). Six more hours later and the hours and minutes are back at 12 but the GMT hand is now at 6 pointing to the 12 on the chapter ring.
ok now if you were in UK and tracking the time in Germany, you'd advance the GMT hand one hour, so assuming it's midnight, you'd set the GMT hand to point to 1 on the chapter ring but if you were tracking the time in California CONUS, you'd need to know what the offset is and set the GMT hand to correspond with that; which is currently -8 and 8 hours behind midnight is 4pm and you'd advance the GMT hand to point to 4 on the chapter ring.
GMT watches are used to show two timezones at once, with some setting the GMT hand to the actual GMT time or their own location. You can then add or subtract to GMT to know the time for other locations in the world.
Hope this helps
