在Gradle中对于JUnit 5测试的配置，可以通过build.gradle文件的test.task的systemPropertyEntries来添加Key-Value对。然后在测试类中使用@BeforeAll和@BeforeEach注释和TestInstance(Instance.Lifecycle.PER_CLASS)注释来引用系统属性。

示例代码如下：

build.gradle

test {
    useJUnitPlatform()

    systemProperty 'myProperty', 'myValue'
}

Test class

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
class YourTestClass {

    @BeforeAll
    fun setUp() {
        val myProperty = System.getProperty("myProperty")
        //do something with myProperty
    }

    @BeforeEach
    fun beforeEach() {
        val myProperty = System.getProperty("myProperty")
        //do something with myProperty
    }

    @Test
    fun yourTest1() {
        val myProperty = System.getProperty("myProperty")
        //do something with myProperty
    }

    @Test
    fun yourTest2() {
        val myProperty = System.getProperty("myProperty")
        //do something with myProperty
    }
}