Gmail blacklisted issue - unable to send to gmail

Hello. My domain and an associated email address has been blacklisted by gmail, the email address was not used for spam emails / similar but we understand that there is an issue with the DNS settings etc. from running diagnostics online. I have full access to the domain, server etc. and we think this should be a quick job for someone who is familiar with Google's requirements, DNS etc. The initial source of the issue was a hacked website, which has since been suspended by us so the source of the issue has been eliminated.

Thank you,