Hugs to you, Josie. FTT is a heart-wrenching label to have for your kiddos. My DD2 was hospitalized at just over 3 months for FTT and it was the hardest experience of my life.
She is in the 2nd percentile now, at 19 months, and only on the WHO chart. She doesn't have any allergies or Celiac as your little one does, so it is a different scenario for us, but I wanted to share what I found helpful.
1. Daily bone broth/home-made stock. The gelatin content is healing to the digestive tract. My DD2 had really severe reflux which led to her feeding aversion. The bone broth was really helpful and she still drinks it. I cook our veggies and grains in stock as well.
2. If she tolerates them, eggs are your friend. I make a soaked baked oatmeal with coconut oil, butter, eggs, and either coconut milk or heavy cream. If you google baked soaked oatmeal a bunch of recipes will pull up.
3. Pates, organ meats. Your DD is older so this may be tricky, but it is worth a try because they pack a serious nutritional punch.
4. I use lots and lots of Paleo baking recipes. Nut flours are calorie-dense and most Paleo recipes require lots of eggs for binding so they are caloric.
5. Carbs!! I fed my DD2 mostly animals products--meats and fats, but we saw a big turn around when we started giving lots more complex starches like root veggies with home-made mayo to dip them in, sweet potatoes with lots and lots of butter, roasted squash, etc...
6. Smoothies with an avocado base. They puree up really smooth and creamy and compliment loads of flavors like berries and banana. I'd add a little ricotta cheese too for extra protein.
7. Raw milk. We no longer have access to raw, but at the time we were really working hard on her healing/weight gain, I had milk from jersey cows (super high fat content) and added 2 TBSP heavy cream for every cup of whole milk I gave her.
8. Fatty treats. Both my kids love butter mints (again, Google has tons of options here) and the multitudes of Paleo coconut oil/butter treats. I mix equal parts coconut oil with coconut butter and the zest of a lime or lemon, and a spoonful of honey and divide it into mini muffin cups then freeze. They make a nice sweet treat with lots of good fats. Home-made ice-cream with minimal sweetener, puddings, custards made with coconut milk.... Yum!
9. Have I totally over-whelmed you yet?
If I serve a grain, like rice, I cook it in stock with a spoonful of gelatin, then I add fat. So, instead of just buttered boiled rice, we have brown rice casserole made with cream, butter, and an egg. Or I make a sauce with caulflower, garlic, parmesan and cream to make basic brown rice more like a risotto.
The ages and causes for our kiddos is different, but I hope you find something beneficial for your family and right away. Hang in there, mama!