Using the plugin on other webpages

It should be possible to use the plugin also on other webpages than Gmail. (As long as they use rich text editors.) For example, the WordPress editor seems to work with this plugin.

To enable the plugin on other pages, add the corresponding pages to the list of includes pages in the Greasemonkey/Tampermonkey options for the plugin. E.g., add https://wordpress.com/post/* to enable the plugin in the WordPress editor.

Your milage may vary depending on the way the rich text editor is implemented.