try using Java 6 , gradle has some known issues with java 7
Add hadoop-core-1.x.y.jar from hadoop distribution to path.
This worked for me in CoPA example. I added to eclipse build path.
Add this to gradle dependencies{} section
compile( group: 'org.apache.hadoop', name: 'hadoop-core', version: '1.2.0' )
It seems like class not found, please check libraries in build path entries.