App Engine with IAP is an extremely simple and powerful way to securely manage applications created for internal use of an organisation. This video explains why and how it works.

Further reading
The Github code from Google: https://github.com/googlecodelabs/use...
Codelabs appengine auth with IAP (step by step tutorial): https://codelabs.developers.google.co...
user-auth-with-iap#0
What I leant this stuff from (very good video):    • Centralize access to your organizatio...  
IAP getting user's identity: https://cloud.google.com/iap/docs/ide...
AppEngine VPC SC limitation: https://cloud.google.com/vpc-service-...