Use the GD > Post Meta block or shortcode with the post_author key. It shows the name of the user who owns the listing, linked to their author page.

[gd_post_meta key="post_author"]

1. Find the template you want to change. The details page template is set in GeoDirectory > Settings > General > Pages > Details Page, and the archive item template (each listing in a list) is the Archive item page on the same screen.
2. Edit that page and add a GD > Post Meta block, choosing Author as the key, or paste the shortcode above.
3. Update the page and view a listing.

By default it shows an icon, the label “Author:” and the link. Add show="value" to show only the link. The link text is the user’s nicename, the URL friendly form of their username.

[gd_post_meta key="post_author" show="value"]

Where the link goes

The link uses the normal WordPress author URL. On a plain WordPress site that is the author archive. If UsersWP is active, it sends author links to the user’s UsersWP profile page, unless “Disable author redirect to UsersWP profile page” is ticked in the UsersWP general settings.

GeoDirectory itself has no user profile pages, so UsersWP is the usual way to give listing owners a public profile.