How is this possible? I can hear you think. GMT is Greenwich Mean Time. Greenwich is located in England. Greenwich is located in Greenwich. All of England is in one time zone. So how is it possible for Greenwich Mean Time to be one hour behind London?
It's because England is on Daylight Savings Time. Isn't that brilliant?
(All of this lovely silliness came up because I just had to figure out how it was that Rally Argentina was running at GMT -3 but was still four hours ahead of me, since I know that I'm eight hours behind England. Yes, I know the rally ended over a week ago. It's still been bugging me. Isn't it fun being anal retentive?)
On a related note, this site is pretty cool for figuring out what time it is anywhere in the world.