It sounds like it's time, then. I wonder what is triggering the seizures. How long have you had her? How old is she? You mentioned the dent in her skull, I wonder if it is some sort of old injury that caused brain damage..
Again, I am terribly sorry.
As for the euthanasia, I have found the cool down/freezer method to be the best option for something as small as a gecko. They are totally unresponsive when pulled from the fridge (no reaction to pokes, prods, no eye movement, appear to be asleep. I don't even notice respiratory movement.) and freeze quickly due to their small size. At 22g it won't take long. I would, too, would be concerned about the stress involved at the vets office as well as the accuracy of the needle to the heart/pain involved beforehand.