GMT is just the forum default, you can customize the times that you see on the site by going into your user contol panel (click on the "user cp" icon at the top of pages), then click on the "edit options" link - and on that page there is a pull down menu labeled "Time offset" where you can configure the forums to display in your local timezone.
Here's a direct link to the edit options page:
http://130.94.21.174/gcforums/member...on=editoptions
EST is GMT -5. Once I figure out how to set the default time for the entire forums I'm going to set it as EST. Your custom settings will overide this if you have it set for a different timezone.