Hi,
More details are required to recommend specifically. There are many reasons behind not gaining weight, that reason first need to know. You can follow protein rich nutritious diet, which contains good quality and quantity protein like egg, dahi/ yoghurt, paneer, soyabean, sprouts, almond, cashew etc. You should increase your food intake gradually.
Next Steps
To get personalized diet chart you can consult with me on practo.
Health Tips
You can have calorie rich one dish meal like, khichdi, fruit custard, high protein milk etc, which will add more calorie in small quantity food.