If you want to take out a subscription on your Android device, you will need a Google Play Account, on which you have entered your credit card details. In-app-purchase will not be possible with a gift card.
Here is the step-by-step process:
1. Go to Shop in the Zattoo app (the menu that is displayed when you swipe your finger across the display from left to right)
2. Select the appropriate period (1 month or 12 months)
3. Confirm your Google password
4. Confirm your Zattoo HiQ purchase
5. The following message now appears at the top of the screen: "You have been enabled for one month".
Please note that Auto-renewal is activated for each Zattoo subscription purchased. This can be managed in your Google account.