OK I found how to get the C-Gear skin! After you've done the mini game and everything (as explained here) you need to go to Promotions first and there's a link for the Hero C-Gear.
Click that, the click Submit then OK. Then go to Customize and choose the Hero skin.
Do Game Sync in your game and you should get the new skin! If you already woke up your Reshiram then you might need to tuck a Pokemon in again to get the skin.