The first place to start is with the documentation. In particular, the user guide contains a lot of reference material. Also, keep in mind that Gradle build scripts are simply Groovy code that drives the Gradle API, so you might find the API documentation useful.
If you don't find your answer in the documentation, you might try the mailing lists. You could also join us on IRC and ask your question there.
Here is where you can find out more about contributing to Gradle: