Probably because UTC (the setting of time that I see this setup uses - Cordial Universal Time) is the same as GMT when it's not in British Summertime (which is sometime in June, if anyone was curious).
The bottom of my screen says this:
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group :: All times are UTC - 8 hours
So I'd say that phpBB3 doesn't go by DST, probably because many areas of the world don't go by it. (The -8 hours is just to account for the time zone I live in.)