Edit the file /etc/TIMEZONE and set TZ=GMT0. This will be the default TZ for the system but might be overriden in the .profile of each user. It's not unusual for a UNIX to serve users all around the globe with different TZ's for each. In a very real sense, you don't have to do anything because the system always is in UTC (GMT, almost) and simply counts seconds since Jan 1, 1970 00:00:00; TZ only controls how those seconds are displayed.
If you want to make certain that all daemons and system processes receive the corrrect TZ setting then you really should reboot after editing /etc/TIMEZONE.
If it ain't broke, I can fix that.