The documentation from Google is not really clear about it. Here ( https://cloud.google.com/compute/docs/vpn/overview ) it says
Google Cloud VPN secu
Google App Engine standard CAN'T use VPN directly, however the new Flexible Google App Engine (now officially released, no longer in beta) CAN use it directly. Just set up google cloud VPN and your network from the console and make sure to include the network in your app.yaml file.
Flexible App Engine is be the easiest way for new applications to utilize VPN connections since there won't be any migration headaches.
For a standard app engine application that can't be ported over to flexible for whatever reason, you CAN still use a VPN connection - however, it requires a service to be running on flexible app engine (or compute engine), and your app will need to interface with it somehow. This may be the simplest method, as migrating from standard to flexible is not the simplest of tasks. I know - I've done it.