Put 3 cups of water and all the sliced ginger in a blender and blend until ginger is chunky smooth.
Pour the blended mixture into a large saucepan. Stir in the remaining 3 cups of water and the sugar, and bring to a boil over medium heat. Reduce to low and simmer for 30 minutes.
Allow the syrup to cool in the pan.
Line a sieve with two to three layers of cheesecloth and strain syrup. Squeeze the cheesecloth bag tightly to get out all the gingery goodness.
Pour syrup into containers and refrigerate or freeze.
Makes about 8 cups of very strong ginger syrup. Ginger syrup should be refrigerated in an air-tight container. Properly stored, ginger syrup will last about a month. You can also freeze ginger syrup. Frozen, it will stay good for 6 months or more.