iX Magazin (german) on multi-core programming with overview (by Johannes Link) and Dataflow concurrency (by Dierk König) explained with GPars examples.
Groovy in Action, 2nd Edition chapter 17 (available in MEAP) covers the basics of GPars.
Wrote or spoke about GPars
JVM Concurrency and Actors with GPars article at Dr.Dobb's
Flowing With the Data - Václav Pech
Testing Asynchronous Code with GPars Dataflows by Hamlet D'Arcy
JaxEnter short article (german) – 2009-10-09