I have gone through your post in which you have asked about Nutrigain.
Nutrigain is a product made of certain herbs which are helpful for gaining weight.
But these products are only dietary supplements & should not compromise on routine healthy food such as:
1. Consume more of natural diet like fresh fruits, green vegetables, milk, eggs, wheat, etc.
2. Avoid oily and junk food.
3. Stay away from smoking
4. Best is to stay active, exercise regularly, avoid stress
, get good sleep, think positive and stay happy.
And as you have mentioned about the past history of chicken pox
& presence of kidney
stone in both the kidneys, it will always be better to consult your treating physician before taking anything.
Wish you a healthy life.