GMT doesn't have a summertime change, more info below.
"Jack
iGP operates on a constant time zone (GMT/UTC) that doesn't move with any local DST anywhere in the world. Because the audience for iGP is international and most leagues are a mixed bag, if we followed one local DST configuration then we'd alienate the rest of the world every time the clocks moved.
If the new race time is inconvenient when your local clocks change then it's at the hosts discretion to decide whether enough people are of one timezone to move the start time.
Quoted from:
http://igpmanager.com/play/?url=forum-thread/2627#18736