In a 1-quart pan, mix sugar with 1/2 cup water and stir over low heat until sugar is dissolved, about 5 minutes. Let cool or chill.
In a 3-quart pitcher, combine gin, lime juice, and the cooled syrup. Cover and freeze until ready to serve.
To serve, add soda water. Pour into ice-filled glasses and garnish with lime.
Note: Nutritional analysis is per serving.