It's good that your GP is being pro active in refereing you to a pead as so many don't.

It is good that baby is still gaining but the amounts need to be more, slow weight gain is 1 thing as long as they are following curve on the growth chart BUT to drop 4 centiles on chart is a concern, it might be the fact she is using energy to fight infection etc or it might be some under lying issue, you have allready mentioned your intolerance to gluten, may be that may be coeliacs or something else.

No harm in keeping a food diary and its a very good idea to chat with your health nurse

Good luck xoxo