I am not familar with CG menu but if flatbreads are on the menu I don't see an issue of having just cheese on it.
Also if salmon is on it with sauce, they can always hold the sauce and serve it plain.
Most of the restaurants will do this.
You mentioned special needs, if you feel comfortable discussing that on a forum, if you post in the disabilities forum the foodies over there can guide you better.
But I am sure there would be a way for you to do what you wish, but you may have to call or put it on your reservation in some way.
Good luck, hope you get to dine there.