Gradle Merge Junit Report

You should be able to use the Ant task JUnitReport to achieve your goal. Something like the following should work:

configurations {
    antJUnit
}

dependencies {
    antJUnit 'org.apache.ant:ant-junit:1.9.7'
}

task mergeJUnitReports {
    ext {
        resultsDir = file("$buildDir/allreports")
        targetDir = file("$buildDir/test-results/merged")
    }

    doLast {
        ant.taskdef(name: 'junitreport',
                    classname: 'org.apache.tools.ant.taskdefs.optional.junit.XMLResultAggregator',
                    classpath: configurations.antJUnit.asPath)

        ant.junitreport(todir: resultsDir) {
            fileset(dir: resultsDir, includes: 'TEST-*.xml')
            report(todir: targetDir, format: 'frames')
        }
    }
}

Keep in mind that you will have to declare a repository to allow Gradle to resolve the ant-junit dependency.