If you're trying to put on weight I suggest https://www.reddit.com/r/gainit/wiki/index and the subreddit itself. Generally the upvoted posts / comments have good advice. Also see https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-h...

To answer your questions:

- Red meat - we don't know (see https://www.reddit.com/r/nutrition/comments/ldaitn/is_red_me...)

- Fat is good for you, and saturated fat / cholesterol is fine unless you have certain genes that make you susceptible to heart attacks. Regular checkups / lab work should reveal this

- A gallon of milk a day isn't good, try to find more balanced ways to get your calories in

- Fast food is fine occasionally (e.g. once a week) but again, try to eat less processed high-calorie foods instead

Some examples of high-calorie whole foods: nut butters, fatty fish, cheese, yogurt, whole grains, potatoes, foods with with olive oil / butter

> It seems like what is conventionally known to be 'healthy' food also happens to be low-calorie food. I wonder if that is mostly because a lot of people are overweight, and so for them whatever makes them consume fewer calories is healthy?

Yes. What's healthy for a skinny person vs. an obese person vs. a sedentary person vs. a marathon runner is all different. For a starving person, even fast food and donuts are better than nothing. But, high-calorie whole foods are even better, because they have micro-nutrients as well as calories.

Lots of conventional diet advice (e.g. low-calorie = healthy) is geared towards the average American, and sadly lots of conventional diet advice sucks.

What about half a gallon of milk every day and some carbs and nothing more? It is known that a great mathematitian Grigory Perelman used to follow exactly that kind of nutrition for years.

You can get away with a bad diet for many years.

I believe it, but the question is what bad in this diet. Not balanced is too general answer.