From what I've found in my internet research, these are the steps you have to follow to get Garchomp:
1 - Make sure you actually participated in the Sinnoh Classic. 2 - Go to http://3ds.pokemon-gl.com/ 3 - Log in (you should have had to log in to register for the competition anyways) 4 - There should be a little notification or something. 5 - Click on that and it should give you your Garchomp code 6 - Enter the code in the "Receive Mystery Gift" screen 7 - Obtain butt-kicking dragon shark.
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