When I unlock achievements on Xbox LIVE, I get rewarded with Gold. Here is an example of Gold giving achievements in Wordament worth 10-20G:

![Achievements](https://i.stack.imgur.com/8AMWd.png)

Collectively, from different games, they accumulate on my profile:
![Gold](https://i.stack.imgur.com/iKVwy.jpg)

Where can the Gold be used?