You need to test the GoPay payment system before Processing “Live Transaction”. (GoPay can ask your Notification URL, it can be found on the plugin Settings Page).
Note: Find out more information about GoPay SANDBOX Testing: GoPay FAQ.
Login to your test account by this link SANDBOX with your Test Personal Account Name and Password.
Now you can make Test Purchase to Test the Gateway with Card No. 4188030000000003 any three or four digit CVV Number and along with any future month and year for Expiry Date.