hai,
Taking high calorie food with lessening your physical activity will gain you weight.
foods like milk and milk products,jaggery,potato,almond,pista,ghee,potato,mango,
arrowroot,Dal prepared with ghee,peanuts,rice,ice cream,lassi,chocolates,urad dal preparation will gain you weight.
if you eat non veg take meat,fish frequently.
it will help in gain weight consistently in a healthy way.
if you need to gain in a short period
steroidal injection is available. but it is not appreciated for normal weight gaining purpose.
thank you
hope i answered your query