GMT stands for Greenwich mean time. GMT timezones start at GMT 0(United Kingdom, and other nearby areas) and go up or down, depending on which direction you go. East= +, West= -.
It's a simple matter of adding or subtracting. Just take the GMT time and add or subtract the appropriate amount of time, depending on your Timezone.
Ex. Subtract 6 hours from the GMT time if you live on the US East Coast, or subtract 8 if you live on the west coast.
You can find a list of Timezones and their areas on Google.