I got all of the Pokémon on Cynthia's team on my Platinum version before I restarted it, bred them, and sent the eggs (I think, or maybe I waited until they hatched) over to my SoulSilver game, and released all the Pokémon I had when I was finally able to trade. Since I decided that I would have all Cynthia's Pokémon, naturally, Garchomp ended up following me around the majority of my journey, and continues to do so.