In a small saucepan over medium heat, bring blueberries, granulated sugar, and water to a simmer. Reduce heat to medium-low and cook, stirring often, until most blueberries have burst, about 5 minutes. Transfer to a small bowl and refrigerate until chilled.

Divide 4 tbsp. blueberry syrup between 2 ice-filled glasses. Add gin and top with tonic water. Add more blueberry syrup or tonic water to taste. Garnish with a lime or lemon wedge.