Gradle like virtually any other object in Gradle and also all objects that Gradle creates for you from plugins and so on are decorated automatically to be ExtensionAware. Only a handful of objects actually declare being ExtensionAware explicitly, but you can cast practically any object to ExtensionAware and then use it. In Groovy the casting is simply not necessary due to its duck-typing and you can just use it as ExtensionAware.
Another way of sharing that JPA version would be to have it in gradle.properties, because then you can access it from settings script and build script.