According to http://checkstyle.sourceforge.net/anttask.html the Checkstyle task can take a config attribute which can be a file, BUT it also can be a relative location which it’ll lookup using ClassLoader.getResource(). The Gradle checkstyle task though enforces that the input be a real file.
Additionally the checkstyle task (via Ant) supports a URL, and Gradle does not expose it that field or possibility.
This should be a bug that is tracked.