Bombay Sapphire is 94 proof (47% alcohol) -- so it's a bit more caloric than standard 80 proof (40% alcohol) spirits. A 1 oz serving of Bombay Sapphire has approximately 78 calories (versus approximately 67 calories for 80 proof product).
There are about 100 calories per shot of gin. There are 0 calories in club soda, as opposed to 83 calories per 8 oz of tonic. If you want fewer calories, choose club soda.