<<Disclaimer:  Verify this information before applying it to your situation.>>
 
>Is anyone aware of weight gain as a symptom or any accepted medical
>theory as to why a person suffering from Celiac Disease would gain
>weight?
 
Accepted medical theory...no.
 
However, it has been suggested that people respond to an "allergy"
in two different ways.  Either they 1) eat more of the offending
food item because it sets up a craving, or 2) they eliminate it
from their diet because it doesn't make them feel good when they
eat it.
 
Assuming #2 is plausible, your daughter may have been eliminating
[unknowingly] glutenous foods from her diet.  But what has she
been substituting?  I can see two possibilities -- fats or
fruits and vegetables.  If her choice is fats, there's your answer.