Hi Rogerio,
well I haven't a deep knowledge of Google App Engine but AFAIK it's a framework that leverages standard Web components like
Servlets, JPA,
Java Mail, etc. So as long as you are using standard Java EE components in your Google App Engine it won't be too difficult migrating your applications to JBoss/WildFly. Other components like JDO still can be installed in JBoss/Wildfly but that's not out of the box. see for example:
https://community.jboss.org/wiki/GettingStartedWithJDOOnJbossAS7InSevenSteps
Hope it helps
Francesco