I was diagnosed with celiac at the age of 18 months, im now 18 years old. im 5'10" 180 pounds, mostly muscle.
When i was young i was always a smaller kid but around 14 i started eating more protein foods and working out.
My best suggestion is try to just get a good amount of protein in your diet and work out, such as.
Protein Shakes(Whey protein)
chicken...