Gradle Geb specific browser

Currently I'm trying to automate my tests with Geb and Gradle and using Firefox the whole time. Now I'm at that point that I want to choose between different browser like Firefox, Chrome and Browserstack and/or run all tests with all browsers.

It sounds easy but I can't get the right solution. I read a few examples like: http://ift.tt/1Sd1ilO I created a few tasks like firefoxTest and chromeTest. At this point I'm ignoring the BrowserstackTests, Now the funny thing ist that if I run the chromeTest it's starting Firefox and I can't find a explanation who to choose between tests and I can't figure out thow does it the example from above.

Here is my current build.gradle.

def gebVersion = '0.13.1'
def seleniumVersion = '2.51.0'

apply plugin: 'groovy'

repositories {
    mavenCentral()
}

dependencies {
    testCompile "org.gebish:geb-spock:$gebVersion"
    testCompile("org.spockframework:spock-core:1.0-groovy-2.4")

    testRuntime "org.seleniumhq.selenium:selenium-support:$seleniumVersion"
    testCompile "org.gebish:geb-junit4:$gebVersion"
}


task firefoxTest(type: Test) {
    dependencies {
        systemProperty "geb.build.reportsDir", reporting.file("$name/geb")
        testCompile "org.seleniumhq.selenium:selenium-firefox-driver:$seleniumVersion"
    }

    systemProperty "geb.build.reportsDir", reporting.file("$name/geb")
    systemProperty "geb.env", "firefox"
}

task chromeTest(type: Test) {
    dependencies {
        systemProperty "geb.build.reportsDir", reporting.file("$name/geb")
        testCompile "org.seleniumhq.selenium:selenium-chrome-driver:$seleniumVersion"
    }

    systemProperty "geb.build.reportsDir", reporting.file("$name/geb")
    systemProperty "geb.env", "chrome"
}