The cause for her over active gag reflex is probably the cerebral palsy.
It is well known that people with cerebral palsy have abnormal reflexes, and the cerebral palsy patient will also have a hyper-active gag reflex.
Anxiety due to OCD only heightens her gag reflex even more.
Talk with some neurologist or dentists about medication treatment using sedatives, anesthetics and other drugs that will prevent it.
Hypnosis is the other possible therapy.
Consult her physician for OCD treatment with cognitive-behavioral therapy.
Best wishes!
Marija