ant.java使ってたけどJavaExecだけでも良いっぽい

apply plugin: "java"

repositories {
    mavenCentral()
}

dependencies {
    compile "org.apache.zookeeper:zookeeper:3.4.5"
}

task runClient(type: JavaExec) {
    classpath = sourceSets.main.runtimeClasspath
    main = "ClientApp"
}