You may need to find another functional medicine doctor that knows how to treat and not recommend a specific diet. You are unique and your gut lining and microbiome need specific supplements to build back up to rid you of whatever you have been diagnosed with. It took me three tries before I found the right functional medicine doctor. And the nutritionist I worked with had my results and guided me with what foods I needed to eliminate for a period of time and what kinds of food are good for the gut - examples: fermented foods (pickles in the refrigerated section of the market, kefir, sauerkraut), omitting fruits high in sugar (bananas, oranges, apples) and consume berries of all kinds, concentrate on organic sad much as possible, use grass fed meats, dairy products, wild caught food. Over time, food in the US has been inundated with too many chemicals. You need to also eat as cleanly as possible. Read labels - I do all the time. I get an almond butter that’s just almonds, no added sugars or stabilizers or added preservatives. It’s more complicated than just going on some diet.
Hope this is helpful.