Hmm, I’m convinced there must be something going on with your WordPress installation/server.
Tell your hosting company that you’re running GeneratePress 1.3.14, but the latest version is 1.3.28 (shown here: https://wordpress.org/themes/generatepress/)
All plugins and themes on WordPress.org are updated through them directly to WordPress, so if the update isn’t showing up, it means your site isn’t communicating with WordPress.org.
It may be worth checking your wp-config.php file for any lines that look like they deal with updates as well.
I’ve never actually come across this issue before, so it’s tough to diagnose what’s going on.