The reason for this should be that Main-Class is not specified in the manifest. Either write a bat script and call the jar command to specify it, or use jar to specify it when building gradle

jar {
    from {
        configurations.compile.collect {
            it.isDirectory() ? it : zipTree(it)
        }
    }
    exclude 'META-INF/MANIFEST.MF'
    exclude 'META-INF/LICENSE'
    exclude 'META-INF/*.SF'
    exclude 'META-INF/*.DSA'
    exclude 'META-INF/*.RSA'

    manifest {
        attributes "Implementation-Title": "..."
        attributes "Implementation-Version": version
        attributes "Main-Class": "..."
    }
  
}

Use jar task packaging in gradle