Many people who do Starting Strength and drink a gallon of milk a day do it because it works. Their goal is to get bigger and stronger. If their goal were to stay or look athletic, they'd do something else. It's quite possible to get a lot stronger without getting bigger, but it's so much easier to do it while getting bigger.
And drinking a gallon of milk a day in the short term doesn't make you look like a fat slob unless you were fat to start with. Personally, I wouldn't drink a gallon of milk a day beyond the first month. Then, I'd probably lower my calorie intake a little because of diminishing gains.
But the bottom line is, if you just want to get strong, SS + GOMAD is a good option, but if keeping your weight down and looking like a model year-around are your goals, then do something else.