The potatoes + milk + protein powder plan depends entirely on your metabolism and activity level. No one can tell you it will work based off the limited info given.
I’d eat a lot of protein, at least 1.5g per lb of weight.
If you are carb tolerant (which i’m assuming you are because you listed potatoes and milk) then eat your protein first in each meal, then use the carbs to take care of any left over hunger. This can help ensure you are able to eat all the protein you need.
If you’re a fat-centered person (use fat for energy) the same guideline can apply as above, just substitute fat for carbs.
Eat a lot but don’t rush into it. I’d guestimate your caloric needs based on your weight, activity level, metabolism, etc., then figure out at least 1.5g/lb body weight in protein and then use the rest for carbs/little fat or mostly fat (if using fat for energy).
Increase your cals a little bit if you don’t gain weight after a week or two.
All that and getting stronger in the gym is going to lead to growth.