This plugin is not listed on the WordPress Plugin Repository, but on GitHub. It allows you to add a post author without the need to register them as users in your site, so there is really no need of it when using Molongui Authorship, which already provides that feature —and many more.
If for any weird reason you still want to use that plugin, know that author pages will list every post on your site rather than only those authored by the user being displayed —as it is expected.
So, really, disable and uninstall that Guest-Author plugin. Molongui Authorship already features a guest author functionality, so there is no need to use another plugin for that matter.