I\'m trying to deploy one of my java apps in GAE and I have used MongoDB. Unfortunately GAE doesn\'t allow me to create databases, however it suggests using Google Datastore
I think you could deploy your app on app engine and create a separate vm instance in google compute engine that would have mongo installed, then you plug your GAE app on that vm for the db connection. This would allow you to keep all your app in the google cloud. See https://cloud.google.com/solutions/mongodb/ and http://docs.mongodb.org/ecosystem/platforms/google-compute-engine/