If only it were that simple
You know when you’re not hungry? Eat an ice cream or 2.
Anyone can gain weight.
Are you specifically trying to just gain muscle? If so, eat lots of steak and potatoes and lift weights.
Skinny people stay skinny because they don’t eat.
This exposes how much popular misinformation there is about dietary science.
The fix to being skinny is never as simple as “eat more” in the same way the fix to being overweight is never as simple as “don’t eat.”
There are a whole bunch of psychological, physiological, and downright safety concerns that go into effectively dieting (yes if you just start starving yourself you can die). And on top of that, research has repeatedly demonstrated that fat/skinny shaming just makes people worse off than they were.
I encourage folks to have the openmindedness to understand that if things were really this easy, no one would have weight concerns.
It does boil down to eating more or less. It just might not be as easy to learn to do that