On gabapentin -- it is not used for epilepsy in cavaliers. generally the usual drug is phenobarbitol, or bromide or diazepam or some mix.
Some dogs may only ever fit once or twice or not enough to need a regular medication. Often something is just given during a seizure.
Perhaps to consider are conditions like episodic falling or syringomyelia, though it is uncertain if fits are connected directly to SM. If diagnosis is uncertain or you want to talk to a neurologist I would definitely recommend Clare Rusbridge. I don;t know how often epileptic dogs would scream -- to me that would possibly indicate something with pain like SM, and not an actual fit but a serious pain episode. These can be confused with fits. If it happens again try to get it on video, as hard as that may be -- it will likely help with a diagnosis. Though your vet will surely try blood tests for epilepsy and other elimination tests for various things.
That staring and shaking head is actually common in cavaliers and seems to usually be harmless. I know several neurologists have noted this now. So it may or may not be connected. Or the shaking and the rest could be vestibular disease -- though don't think that is usually connected to screaming fits though it can appear to be like a stroke has affected the dog.
Please let us know if you learn more.