I don't think target and source compatibility make sense:

sourceCompatibility = 1.8
targetCompatibility = 1.8

I'm also not sure about the configuration of Eclipse's classpath, but I'd have to look at Kotlin's Eclipse plugin more closely to be sure.

eclipse {
    classpath {
         containers.remove('org.eclipse.jdt.launching.JRE_CONTAINER')
         containers 'org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-1.8'
    }
}

If the above isn't needed, then it might also be possible to stop applying the Eclipse plugin too.