GMT is five hours advanced from Eastern Standard Time; that is, 0800 GMT is 3 AM EST. But of course during the (extended) summer hours, we are not on standard time, but advanced one hour, so the difference then is 4 hours. Kind of the same idea as British Summer Time, but the start and end of that period is different in the US from the UK. In fact every once in a while they change when EST changes to EDT too!

It takes me a while to figure it out too.