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)
hamburgers/chicken burgers/ turkey burgers
yogurt( if not dairy intolerant)
Cream of rice(very good in morning for those complex carbs)
sandwiches with gluten free bread of course( i use olive oil instead of mayo)
gluten free pasta
there are tons of foods out there, yes they are more expensive but if you wanna stay healthy its the way to go.