It's a way to refer to GMT, Greenwich Mean Time. Navy and civil aviation uses the letter "Z" (phonetically "Zulu") to refer to the time at that prime Greenwich meridian.
There are another, for example, to U.S. time zones:
Eastern: R, Romeo
Central: S, Sierra
Mountain: T, Tango
Pacific: U, Uniform
Alaska: V, Victor
Hawaii: W, Whiskey
Today UTC, Coordinated Universal Time, is more accepted than GMT