We can give you a couple pointers, but if you need more help you could contact a GD Expert for hands on support: https://geodirectoryexperts.com
GD should work with any theme, so, pick one you like and if there are layout issues let us know and we will help with that. People choose all kinds of different themes and that’s ok.
Things to do:
Backup your site
Clone your site and set up the new site as a place to test new themes
Make notes about what changes must be made to markup
Switch some markup from shortcodes to HTML that will run under any theme
Once you choose a theme, make a new clone of the site
Decide whether you will work on a development/staging site or execute changes on the live site. We definitely recommend working on a development site.