IaaS = Compute Engine
Google App Engine has two environments: Standard and Flexible. At a high level, the Standard environment is a sandbox that supports a specific Java version, intended to be a low cost or even free runtime that is highly scalable. The Flexible environment on the other hand is a better fit for dockerized applications with additional dependencies on the environment.
| Standard environment | Flexible environment |
|---|---|
| Limited free tier. | No free tier. |
| Java 7 only. | Java 8 only. |
| Sandboxed, supports a subset of Java libraries. | Docker-based, any Java package that works on Debian Linux. |