As per the official GCP document :

“You can switch a Public NAT gateway from automatic NAT IP address allocation to manual NAT IP address assignment; however, the NAT IP addresses cannot be preserved. Even though automatically allocated NAT IP addresses are static, they cannot be moved to a manual NAT IP address assignment.”

Public NAT also automatically removes a NAT IP address when it no longer needs any source ports on that NAT IP address.

So it is not possible to reserve an ephemeral IP address used by Cloud NAT.

So try changing the IP to manual and keep your IP address, but you have the rework of changing your systems and communicating the changes to customers.

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