Tried my first VPN to GCP and didn’t have much luck with IKEv1.  While it did detect the remote router being behind NAT, Phase1 wouldn’t come up due to an ID mismatch:

received NAT-T (RFC 3947) vendor ID
remote host is behind NAT
IDir '' does not match to ''

Where is the Real private IP of the router and is the public NAT IP.

See also the same configuration with IKEv2.