How can you safely reintroduce foods?

I’m still in the midst of my Functional Dyspepsia but I was wondering how do I even reintroduce foods to my body without worrying about or getting a gastritis flare. For the past 5 months I’ve only eaten rice, bananas, applesauce, potatos, and egg whites. I’m very malnourished and have deficiencies because of this. When I try adding new foods my body has a problem. Even drinking a sip of Gatorade is a major shock. My body is not had processed foods in months, so I don’t even know where to start.