See this Gradle project: https://github.com/emosesSfdc/groovyBugReproCase.
LibraryTestBase is a Java class with a private method
LibraryTest is a Groovy test that extends LibraryTestBase.
When a Java stub is generated for LibraryTest, it doesn't have an implmentation of getProperty (from the GroovyObject interface), so java compilation of the stub fails.
I first saw this when I updated a project from Gradle 2.3 to Gradle 2.4; I have a test class which extends org.glassfish.jersey.test.JerseyTest (in jersey-test-framework-core:2.12), which has a private getProperty method. There was a change which made in Gradle is so that now, Java stubs are generated in circumstances where they weren't before, and my compilation fails.