This is an issue with your credit card and our processing platform - Braintree.
After the 'gateway rejected: AVS error" message there should be a letter code in brackets. The letter explains the reason for the error.
Here is a complete list of the AVS errors with their corresponding letter explaining the issue:
AVS & CVV Response Codes
If there is no AVS listed and the error simply reads "Gateway Rejected" or "Gateway Declined", this means that your bank has rejected the transaction.
Reach out to your bank and they'll be able to help you resolve whatever is causing the issue!