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 in brackets explains the reason for the error.
Here is a complete list of the AVS errors with their corresponding letter explaining the issue; https://developers.braintreepayments.com/reference/general/processor-responses/avs-cvv-responses#avs
If there is no AVS listed and the error simply reads "Gateway Rejected" or "Gateway Declined", this is indicating that your bank has rejected the transaction and may be due to insufficient funds (but can be from other causes). Reach out to your bank and they'll be able to resolve whatever is causing the issue!