Sounds like you need to allowlist google from your grey list. They have a long retry on their side. Once you tell a server to 'go away, come back later' it is really up to the server to decide when or if to retry. Additionally, If they use multiple sending IPs, you can end up grey listing again and again before they try back with a good ip.

You'd either need to allowlist the big providers sending blocks or just drop grey listing all together.