test task that prints out environment variables and those set for JVM executors that gradle will run and shows how to set specific systemProperty from external value...

build.gradle

apply plugin: 'java' //needs this to have test task defined

test {
    println "Host system properties"
    System.properties.each { k,v->
        println "$k = $v"
    }
    systemProperty 'db.test.mode', System.getProperty("db.test.mode", "EMBEDDED")
    println "Gradle system properties"
    systemProperties.each { k,v->
        println "$k = $v"
    }
}