For gaining weight diet should be proper & yoga is to be done two times a day if possible with empty stomach some kriyas like kunjal help in it ,
Surya namaskar is enough for this but 1st week 5 Surya namaskar 2nd week 10 so every week you need to increase 5 till teacher 50 if you want stronger body than go till 100 it will show you tremendously changes diet in next lecture what to eat & when to est