Hello.
App Engine is regional [1] and the app should be available across all zones within the region you choose. However, it is not possible to change the app region after you set it.
App Engine does not have the same high availability mechanism present in Cloud SQL, so, you would need to build your application with that in mind. That said, I recommend that you review the following topics which will help you decide the best approach to deploying an application on the Google Cloud Platform.
-- Take into account the App Engine SLA, especially the downtime period [2]. If the SLA is still not appropriate for your use case, it might be necessary to change the strategy and use Compute Engine instead. That would allow you to create cross-region load balancers and forward the traffic to instances in two different regions in case of a region downtime [3][4].
-- As per this document [5], instead of using microservices, where all services are deployed within the same application in one single region, you could deploy multiple services in different App Engine project. However, check the benefits and drawbacks of this strategy here [5].
-- I also recommend to review how instances are managed on App Engine Standard [6] and Flexible environment [7].