In reply to Sonja London.

The sites using the Gutenberg plugin and this sites using the Classic Editor plugin will, by definition, not really overlap much. Why would anyone install the Gutenberg plugin and then install the Classic Editor plugin to disable it?

The Classic Editor plugin is something you use for WP 5.0 to disable the new editor.

The Gutenberg plugin is something you use before WP 5.0 to test out the new editor.

I would expect there is some overlap where some users tested with the Gutenberg plugin and decided they didn't like it, so they installed the Classic Editor to prevent it in the future. I don't see why any of those users would still be actively using the Gutenberg plugin, though.