Actually, similar discussion was done on this Stackoverflow thread[1] and the well-detailed response has the information that I believe you may need. App Engine Standard applications cannot be connected to VPN, however, applications deployed to the App Engine flexible environment can be connected to the VPN.
[1] https://stackoverflow.com/questions/37137914/is-it-possible-to-use-google-app-engine-with-google-cloud-vpn#answer-41516269 -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To unsubscribe from this group and stop receiving emails from it, send an email to google-appengine+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/google-appengine/da088667-9c22-4eb3-b128-394890de9e58%40googlegroups.com.
