Apple AppStore currently powers all Gems purchases. The currency is determined by the chosen country in Apple ID. To change the currency, you need to change a country in iTunes Store.

To do this, please take the following steps:

1. Open iTunes and click on iTunes Store (on the top right corner).

2. On the top left corner click on the Sign In and login to your Apple ID (if you are already signed in, proceed to step 3).

3. Under the QUICK LINKS click on Account.

4. Change the Country/Region under Apple ID Summary.

5. Now open App Store; navigate to Store => Sign out and sign in again.

To find a more detailed information, please follow this link.