Sabrina's Mr. Mime and Espeon are level 53, and Alakazam is level 55, so I would say you're underlevelled. I'd suggest getting everyone up to at least level 52, if not higher. You won't need all of them for the Gym battle, but if you put it off now you'll just have more grinding to do later, and grinding in HGSS is a helluva pain.
I hope this helps, and good luck!