Zulu Time (Z) = Greenwich Mean Time (GMT) and is constant, currently 1 hour behind British Summer Time (BST). Unfortunately over here times for events will often be given as GMT when they really mean BST as many people don't understand the difference, so using Zulu instead should remove any doubt - or at least make people ask "What time's that then?"
In Elite: Dangerous they are using GMT but calling it Galactic Mean Time.
I am used to using GMT/Z being a Radio Amateur as we are required to log contacts using it. As I am typing this the time is 10:04 BST or 9:04 GMT.