$50 Google Play Gift Cards are only available in the United States at this time
You can redeem your Google Play gift card in three ways:
1. At play.google.com/redeem: Scratch to reveal the redemption code on the back of the gift card, and enter that code at play.google.com/redeem - funds will automatically be applied to your Google Play balance.
2. During the purchase flow: After choosing the digital content you want to buy, during the purchase flow, under Choose a payment method, select Redeem a Google Play Gift Card, and enter the redemption code found under the scratch-off section of your gift card.
3. Using the Redeem option on the Google Play Store app on your device: Launch the Google Play Store app, then tap Menu > Redeem, and enter the redemption code from the back of the gift card.