Gaining weight works by putting more fuel into your body than you need. That's physically the only way, there is no way to synthesize energy. Less movement is a bad idea. So more fuel it is. Because of caloric density limits this will necessarily mean that you put a certain amount of stuff in your body (in contrast to the other way around, where physics allows for more options)

What's the job that the weight gain intervention needs to do for you, that any type of food does not? Are you for example disgusted to swallow anything?