Currently, in game gold can be used to purchased two things:

- The Master Skins, which can be bought for 10,000 Gold when you have a Hero at Level 10 which is explained in the [Progression System and Reward Changes blog post][1]:
> At level 10, you’ll unlock the ability to purchase a Hero’s Master
> Skin with Gold, which you can use to show off your prowess with that
> Hero in-game. Just like other Skins, a Master Skin features two
> variations, which you’ll have already unlocked by reaching level 10.
> This means that once you’ve purchased a Hero’s Master Skin, you’ll
> immediately be able to use its variations in-game.

- and the [Piggy Bank Mount][2] which is 20,000 gold.

  [1]: http://us.battle.net/heroes/en/blog/14845424/
  [2]: http://www.heroesfire.com/hots/wiki/mounts/piggy-bank