If a coach ever told me that eating junk food would make me fat, I'd tell my HCs that and wouldn't work with that coach until the situation was addressed. And I just wouldn't stay at a gym that focuses on weight. Period. Gymnasts are in a higher-risk category for eating disorders as it is, and I don't need any pressure on top of that.
But, of course, I also reject the idea that coaches should strictly control gymnasts' diets. There is a huge difference between encouraging healthy and balanced meals, good snacks for practice, etc., and giving out calorie limits and telling kids that they're going to get fat. The latter is completely unacceptable to me.
Oh, also, I should add that diets are important... healthy, balanced, nutritional diets. Weight-loss diets or diets that strictly forbid foods... unnecessary.