Mix the raspberries, sugar and water in a small saucepan over medium heat. Stir until all the sugar is disolved and the syrup is bubbly. Remove from heat and allow to cool slightly.
Put the syrup through a sieve to remove the solids. Chill to room tempurature. You can keep the syrup in the refrigerator for up to 3 days.
To make two cocktails, pour the raspberry syrup into a cocktail shaker. Add the gin, lemon juice and tonic water. Shake to combine.
Fill two tumblers with ice. Pour half the contents of the shaker into each glass. Top with a few raspberries and a sprig of mint for garnish. Serve.