GMT is Greenwich Mean Time and it reffers to the solar time. GMT is set for 0 degree meridian which crosses Greenwich in London. We often use 'let's meet at 6pm (f. ex.) GMT' which means that if you want to meet at that hour, you have to check your country's timezone (by simply Googling 'Canada GMT', though Canada has several timezones so you have to pick the one you live in) and then subtract/add as many hours as your timezone says. So for example GMT -4 means you have to subtract 4 hours from your current time in your place to know what time is in Greenwich.