GCP does not support creating a tunnel from a HA VPN gateway that connects to a Classic VPN gateway. If you attempt to create an externalVpnGateway resource having the public IP address of a Classic VPN gateway, GCP returns the following error message:
You cannot provide an interface with an IP address owned by Google Cloud.
You can only create tunnels from an HA gateway to an HA gateway
or create tunnels from an HA gateway to an ExternalVpnGateway.
This is expected behavior. Create a VPN tunnel that connects a HA VPN gateway to another HA VPN gateway instead.