If it is like other GMTs, you control it just as you would the date, only turning the crown in the opposite direction as for setting the date. And you want to set it based on a 24-hour clock rather than the conventional 12-hour clock that the regular hour and minute hands track.
So, for example, here on the US West coast it is 5:25 p.m., or 17:25 per a 24 hour clock. But my GMT hand, which is set to track my brother's time in England, indicates that it is 01:25 by him.
I usually set the GMT hand with the regular time set on the hour, such as 6:00 or 5:00, so that I can line up the GMT hand more exactly. Then I readjust the watch to the actual time (such as 5:25) and GMT sdjusts accordingly. If I tried to set the GMT hand while the regular time is at 5:25 or similar, I would not be able to align the GMT hand as precisely as I can when the time is on the hour.
By the way, I love GMT hands. Much more useful and interesting to me than chronos.
Cheers.