Digging into the COPYING file and comparing to the codebase, I'm wondering where the GPL-3.0 applies.

This was introduced 11 years ago:

The following licenses cover code other than JRuby which is included with JRuby.

Licenses listed below include:

* GNU General Public License version 3
* Apache 2.0 License
* BSD License
* MIT License

But it doesn't refer to what the code under that licensing is.

Does anyone have any idea what it refers to?