How to attach Gradle source code for debugging (Eclipse)?

Hi there. I have an issue with the JUnitPlatform API in connection with parameterized JUnit4 tests. It’s a problem with the test filter. I want to isolate the source of the issue, before I make a bug report.
I ran my Gradle test task with -DDEBUG and attached the Eclipse debugger. My problem is, when I reach the point, when the JUnit Platform Launcher (org.junit.platform.launcher.core.Root) calls the apply-method on the postDiscoveryFilter, which comes from Gradle (org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor$ClassMethodNameFilter) I have no source code to view in the debugger. Sure I can download the source code but to understand, what’s really going on, I want to attach it in Eclipse - and I have no clue, how. Is there a Source-Jar of Gradle available or do I have to build the Gradle source by myself? I would very appreciate if anyone could give me some hints on this one.