task scalaTest(dependsOn: testClasses) << {
    description = \'Runs Scalatest suite\'
    ant.taskdef(name: \'scalatest\',
            classname: \'org.scalatest.         
        
I do not know how to solve this one, but I can offer you a workaround. Annotate your test classes with @RunWith(classOf[JUnitRunner]), like this:
import org.scalatest.junit.JUnitRunner
import org.junit.runner.RunWith
@RunWith(classOf[JUnitRunner])
class MyTest extends FunSpec{
}
and then, gradle test should work.
Edit:
My dependencies:
compile "org.scala-lang:scala-library:2.10.1"
testCompile "org.scalatest:scalatest_2.10:1.9.1"