Welcome to HCM,
As you are going to gym, is good but one thing keep in mind that initially you may loose weight doing gym as during gym there will be more expenditure of calories and if calories intake is less then there will be negative balance resulting weight loss
So what I believe is first create positive calories balance which will help in gaining weight.
Afterwards when you gain weight then go to gym to shape of your muscles and strengthening of body.
Take High protein diet containing eggs, butter, ghee, cheese, panier, cereals, puddings.
Take enough milk.
You can take moderate amount of fat and carbohydrates.
At this age you may not require any digestive medicines.
ok and bye.