If I was going to be a Gym leader I'd use ground Pokemon as they're awesome plus there is so much variety with them. For example I could have Torterra, Garchomp, Swampert, Nidoqueen, Steelix, Gliscor, Camerupt, Mamoswine, Claydol, Flygon... the list goes on an on. It also means no one could come at you with a certain type and expect victory, they'd have to have a good range of types. They send in Ice I send out Camerupt or Steelix, they send out water I sent in Torterra or Garchomp, they send in Grass I send in Gliscor or Nidoqueen. You see what I mean? The possibilites are endless!