From my understanding, the difference between a 'traveller'/'true' GMT and a 'caller' GMT is which hour hand moves independently.
In a 'caller' GMT it is the GMT hand that moves independently, so you can still move them separately.
Because you'd have the 'local' time set to where you are, and when you want to call someone else, you'd move the GMT hand to their time zone and observe them.
In a 'traveller'/'true' GMT, you are the one changing time zones. So you leave the GMT hand alone, and move the 'local hour' hand to reflect the time zone you are moving to.
So in both cases the hands can be moved independently, but it differs in which hand is moved without hacking/moving the minutes hand.
You can't just 'swap' the hands when assembling either, as that results in the date changing with the wrong hand (it should always change with the 'local' hand).
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