As dessert after dinner..

4 half boiled kampung eggs, approx. 200 calories (or beat into omelette with onions!, approx 300 calories).
1 glass of non-pasteurised fresh milk (Bright Cow's Jerysey Milk), approx. 140 calories
1 Tbsp extra virgin olive oil, approx 130 calories
How much cals. is that? 570 calories, of real food with each and every ingredient having its own health function. Oh, and zero crap.

That product confirm can gain weight? Owh, it just have 1200kcal per serving which you can easily consume with real food, and as DT1 said, no craps

Although body systems as you put it differs from individual to individual, I'd rather say you're among the acceptable range than the rather uncommon outlier.

Don't skip your daily breakfast lunch and dinner. And do not fail to add that as supper everyday. Be 99% consistent for at least 3 weeks. I'm sure you'll see changes.

I would say that you are wrong there. The amount of food that you say that is a lot to you actually aren't because you are underweight and most underweight tends to eat less. And if somehow you ate a lot, that might happen once in a fortnight.

Eat more, each day without a miss. Eat big breakfast, big lunch, big dinner and supper. Unless you are rich, you can consume the mass gainers (which is crap to be honest) and keep on depending on it and once you off, you will lose weight (unless you managed to consume the calorie from mass gainers from somewhere else).

If your goal is to increase your weight and have some meat on your bone, any kind would do. But do you want to be someone with lots of fats on them?