I am able to run Errai on the Google AppEngine Flex environment with no issues, however I am not using 100% of the Errai Framework. Even though erraiframework.org states that the latest version (4.0.0.Final) runs on JEE7, I had to compile with Java 8 to get it working properly (classes such as Marshalling are compiled in Java 8), hence the AppEngine Flex environment and not Standard.
This list (found here: https://developer.jboss.org/thread/233098) is also very helpful to see what AppEngine may support, and what it will not. Items with "no server dependency" should work because it's mostly client/GWT related libraries.
- Errai Bus: Any Servlet 2.x container (or 3.0 for async dispatch)
- Errai CDI container (without distributed events): no server
dependency
- Errai CDI distributed events: Weld >= 1.1.5 and < 2.0
(presently a hard requirement on Weld, not other CDI implementations)
- Errai Cordova wrappers: no server dependency
- Errai Data Binding: no server dependency
- Errai IOC container: no server dependency Errai
JAX-RS in Jackson compatibility mode: no server dependency
- Errai JAX-RS with Errai Marshalling: any JAX-RS implementation plus
errai-jaxrs-provider
- Errai JPA: no server dependency
- Errai JPA data sync: any JPA 2.0 or 2.1 implementation
- Errai Navigation: no server
dependency Errai Validation: no server dependency
- Errai UI Templates: no server dependency