Guest checkout is possible, but only after the invoice has been created, and creating the invoice requires the user to be logged in with the listing having been submitted prior to entering the checkout page.
The option to “Allow posting by not logged in users” is design for use in cases where the user that is posting the event is not going to be managing the listing going ahead. For example, for community events so that visitors can quickly share about an event but not manage the event going forward.
I understand you would like to have that feature changed to do login also, but GD does not have any login related features at this time and there are no plans to add them back in.
To use GD with WPI for paid listings meaning getting the user registered and logged in using either the default WordPress /login.php or with another plugin.
This is made easier with GeoDirectory and UsersWP integration. Are you currently using that? If you are please post questions for support with registration or login to the UsersWP support forum at https://userswp.io/support/