hello and welcome to HCM
I can completely understand your concern.
Firstly I would like to explain you that,you need to get the acidity problem treated first,as if you start eating any protie supplements for weight gain,the acidity would take over,and you wont be able to consume anymore of your diet for the day.
If you were my patient,I would treat your acidity with
Tab.
Razo Once a day,Half an hour before food.
followed by Syp,
Digene 15mins after food,for a week.
and advise you to have small meals in short durations(for eg,in every 3 hours),and avoid long term fasting hours/gap between two meals.
Aftre a week,you can start taking protein supplements
Starting with a minimum and mild ones.
Protinex powder: 2tbsp in one glass of hot milk/water,Twice daily.
Later on,you need to consult your dietician/fitness trainer,to addd up few more weight gaining shakes.
In the meanwhile,if the acidity persists,you can take the medications whenever needed.
Hope this Helped.Good Luck.
Please write for followup queries.
Kindly hit a thanks.
THANK YOU.
STAY HEALTHY.