I think you should go with your guts and switch Drifblim out for Girgantina. First off, they are both ghost type so it benefits you. Though Drfblim is a flying type, Girgantina has access to Fly and other great moves. Also, Girgantina will help you with Lucian, an elite four and Cynthia's Garchomp. Lastly, of your team, Drifblim is your weakest Pokemon on your team. Sure, Gabite is weaker but it will evolve to Garchomp which will then be your 2nd strongest Pokemon on your team.