In a 1-quart saucepan combine the sugar, water and ginger. Set over medium heat and cook, stirring occasionally until the sugar has dissolved, about 15 minutes. Allow the syrup to cool completely, then strain through a fine mesh sieve. Discard solids and refrigerate until ready to use.
Put the raspberries in a food processor and process until pureed. Strain the raspberry puree through a fine-mesh strainer into a clean bowl and set aside until ready to use. Discard seeds.
In a 3 quart pitcher or punch bowl, combine the orange, lemon and lime juices, gin, and seltzer with the raspberry puree and the syrup. Stir to combine and serve over ice in cups.