Improve Cloud VPN to support on premises gateway behind nat
Currently it is not possible to connect a Cloud VPN gateway to an on-premises VPN gateway behind a NAT (https://cloud.google.com/vpn/docs/resources/troubleshooting#gatewaysbehindnat).
This is particularly problematic as changing the network topology or changing the HW cannot simply be done on-premises.
I suggest to support this particular setup to avoid to create a VM with strongswan or similar.
It's easier to setup, maintain and configure.