Spec for packaging and distributing Gradle plugins

### Expected Behavior

Is there any specification about how Gradle plugins get packaged and distributed to the portal? I'd try to write a Maven plugin for packaging and distributing Gradle plugins, but I cannot find any specifications - is it private to Gradle?

Besides, the Gradle API libs don't seem to be available on Maven Central or other repositories. Both look like indicating that Gradle plugin development is private to the Gradle build system. Is that the case?

### Current Behavior (optional)

_No response_

### Context

Described above.