Answer:
The gin is likely to be about 70 calories per 1.25 ounces, and the grapefruit juice is about 80 - 110 calories per 8 oz. (or about 25 calories for the amount that most people would mix with the gin), making an 'average' drink about 100 calories - but, in America, most people would mix 2.5 oz. of gin with 8 oz, of grapefruit juice, making a drink of ~240 calories.