Based on the response from gradle forum post, it seems generate gradle styled test html report is not available out-of-the box with gradle. Gradle TestReport task seems to be dependent on the binary output files generated by "test" task. These are not generated when the tests are run using ant.JUnitTask from gradle. I resorted to "ant JUnitReport task" finally to generate at least a meaningful consolidated report.
test << {
//run ant junit task with reports stored in $buildDir/test-results
//after ant junit completion
ant.junitReport( toDir: "$buildDir/reports") {
fileset ( dir:"$buildDir/test-results" )
report ( format:"frames", todir:"$buildDir/reports" )
}
}
This gives a basic html report. One can customize as needed using XSLT.