Credit cards can be declined for a number of reasons. Respondent usually isn't notified of the specific reason.
Common issues for a Gateway Error:
Your credit card number and billing address are incorrect
Your card doesn't have the available funds, or you've exceeded the limit placed on the card for purchases.
Too many attempts to use the card can appear as fraudulent activity.
Your card is expired.
Contact your bank or card issuer:
If you receive any error message when trying to pay, reach out to your bank or credit card company. Tell them the amount of the transaction, and when you tried to make the charge so they can allow the transaction to be approved.