UTC is coordinated universal time. [  24-hour system  ] 
Abbreviated as "Z"  Zulu.
Short-wave broadcasters, Listeners,
Hams  around the world use the UTC  time
format in their schedules

GMT is Greenwich  Mean Time. ( The time at Zero Meridian. Greenwich ) 

There are  24 basic time zones in the world
Which are separated 15 degrees from both the
west and east of the zero meridian.