If you go to Appearance > GeneratePress > Site Library, and remove the imported site. The site should be restored to the settings you had before installing the site.
The removal process does make changes to your database, so if you have data you don’t want to risk losing, please take a site backup before doing this process. Anything added along with the template will be removed.
If you have a backup of your site from before you installed the plugin, you can also restore that. You may ask your hosting provider if you have a backup of your site from that state. Some hosting providers provide automatic backup services.