Hi there. Sabrina uses Psychic-types, so your best bet is to use Bug, Dark, and/or Ghost-type Pokemon. Ground-type Pokemon don't actually have a type advantage over Psychic-type Pokemon, so you may want to consider finding something else.
Dark-type Pokemon will do especially well at Sabrina's gym since they're immune to Psychic damage, so I'd recommend trying to pick one up. You can get Murkrow and Houndour on Route 7 at night, or pick up a Seedot via headbutt, as Seedot evolves into a Dark-type Pokemon. Afterwards, I'd recommend training at Seafoam B4F until your Pokemon is around Level 55-60.
Hope these tips help - good luck with the gym battle!
Thanks! I have a gastly but I think I'll look for those two Pokemon you suggested. Where would I get head but? :thanks: