As gut irritants and how to strengthen gut health itself i actually have a fair amount of knowledge about this because of my own experience. Fermented foods contain enzymes that help your body make up for the lack of acid.
You don't need those pills. I suggest start having a few teaspoons of apple cider vinegar in a glass of water before breakfast and before dinner. You can also eat kimchi, sauerkraut, and more cabbage into your diet, as well as Greek Yogurt and pineapple. Bone broth is another really easy thing to make and you can pour it on food or just drink it.
The best thing i actually did was make my own fermented cabbage juice. its fking gross but it literally next to changed my life as far as pain and digestive abilities. Its simple to make too. All and all the biggest thing is not eat the foods that will fuck with your stomach. Process carbs, pasta, cheeses, milk all of those in excess will be a pain in the ass and your stomach literally.
As for bulking, again just eat more of the shit you can eat without discomfort. You can get big off just about any food.