Jmaster it's a good suggestion but unfortunately there's something you overlooked: Gyrados isn't Water-Dragon, it's Water-Flying, making it doubly effective against Electric types, and since the final gym leader is Electric and I believe has a Jolteon or a Raichu which can easy out-speed a Gyrados it would not be in your favor to find one.
Another thing you overlooked is the fact that, yes, fire types now do more damage to Empoleon because of the steel-type, but it will be just a normal attack. If I remember correctly, Empoleon's steel type helps more than hurts because now electric attacks do less damage, and even though ground type attacks are now super effective, most ground types (besides Dugtrio) aren't necessarily fast, and Empoleon has a high defense, so you can survive one hit and then knock it out with a move like Surf or Hydro Pump.
Having a Dragon-type move for the final isn't entirely necessary, but you do have a few options (listed in order of what I suggest you do)
1. Teach one of your Pokemon Ice Beam (can be found as a TM, will be doubly effective against Garchomp because he is Dragon-Ground, Empoleon should be able to learn it)
2. Teach one of your Pokemon Dragon Claw or Dragon Pulse (they are both TMs, and I'm almost positive Empoleon can learn Dragon Claw, and Togekiss may be able to learn Dragon Pulse)
3. Raise your own Garchomp (you can find Gible in the Cave under Route 209 I believe, but you'll need Flash).
If I were you I'd think of a strategy for the final gym before worrying about the Champion