Please follow the common Adyen instructions to get set up with giropay.
Adyen has detailed instructions on how to test Giropay.
First, make sure you enable giropay as a payment method in Adyen.
Then, after Gr4vy redirects to the giropay bank selection screen, select giropay Testbank
and
use the following details to make a test payment.
DE48499999601234567890
1234
123456
(for Photo and Flicker devices)