Thanks for writing your query.
There are many preparations available in the market for weight gain. But my advice to you would be that you should not go for these products, even herbal, as they have some or the other side effects.
Instead you should do regular exercise which includes going to gym. This will strengthen your muscles and will contribute towards weight gain.
You should also eat healthy and natural proein rich foods like soyabean, cheese, milk, etc. and foods rich in carbohydrate
like potato and rice (in a limited quantity).
Also get your blood tested for Thyroid
I hope this is helpful to you.