Is gymnastics appropriate for someone with epilepsy?

Depends. Seizure disorders are a problem for anyone wanting to do certain activities. You should seek the guidance of your pcp/neurologist regarding activities to participate in. How well are your seizures controlled? What medications are you taking? How often do you have seizures? What are your current limitiations? Do you drive? Talke to your doctors and make a plan. Best to you!
Risk. The risk is that one might have a seizure while participating in a sport. If the risk and frequency of seizure is high, then one should stay away from sports which are above the ground. Keep your body near the ground and seizure related risk falling from a high bar is eliminated.