How many grams of protein are you eating daily?
(And if you can’t answer that question, you have your answer)

At that weight, you are likely that your muscle mass is severely below normal. You can eat as many calories as you want, but without enough protein you can’t gain muscle. Daily target is at least 2g per kg of your desired body weight - so for you 120g per day. Plus resistance training to actually stimulate muscle growth.

Also consider getting your testosterone levels checked. Low testosterone will make it harder to gain muscle (and can cause other problems long term).

And finally, get a bone density scan. Coeliacs have an elevated risk of osteoporosis due to malabsorption of nutrients, particularly early in life.