How to use jdk.internal package with Gradle?

Question

For example, if I'd like to add an extension on Inet4Address to be able to directly retrieve the underlying 32-bit address as an Int:

import jdk.internal.access.SharedSecrets

val Inet4Address.value
        get() = SharedSecrets.getJavaNetInetAddressAccess().addressValue(this)

IntelliJ IDEA does not show any errors, auto-complete works fine for it, etc.

However, when Gradle compileKotlin runs, I get these compilation errors:

Unresolved reference: access
Unresolved reference: SharedSecrets

This happens despite using a Java 11 toolchain spec in my build.gradle.kts:

java {
    toolchain {
        languageVersion.set(JavaLanguageVersion.of(11))
    }
}

How can I go about giving Kotlin access to the jdk.internal package with Gradle?