What is a GMT function and how does it work?

gmt slot Could you help out and suggest a GMT? I like the idea of multiple time zones displayed on the watch. I can’t say I need one, but it’s sort of cool. I have a set of utility watches (or whatever the proper term is) but the GMT slot is empty.The GMT Standard CNC Lathe Chuck model PHNC for precision metalworking. It can be used for 1st and 2nd operations. The GMT Standard CNC Lathe Chuck model PHNC for precision