Google Play customers can redeem their promotion or gift card codes from a computer in the Play section of Google.com or from a smartphone in the Google Play app. Customers redeeming codes must have a Google Payments account.
To access code redemption from a computer on the Google home page, click on the Google Apps icon, and then select Google Play from the list. On the Google Play home page, click Redeem on the menu, sign in if prompted and enter the promotional or gift code. From a smartphone, open the Google Play Store application and click Redeem, signing in if prompted.