Bundled gems have existed since Ruby 2.2. Before Ruby 2.2, all items that were considered part of Ruby’s ‘std-lib’ (standard Library) were included in the Ruby repository.
Since then, the std-lib now has two categories:
‘Default gems’ are std-lib items whose code is included in the Ruby repo, and they cannot be uninstalled via the gem uninstall command.
‘Bundled gems’ are std-lib items that are not included in the Ruby repo, and they can be uninstalled.
These changes affect CI and other things unrelated to Ruby’s use in SketchUp.
Note that everything was already configured for debugging (launch.json & tasks.json to launch SketchUp in debug mode). I didn’t have to uninstall or deactivate any other plugins.