This isn't strictly a GMarks issue, but a bug with Google Bookmarks that affects GMarks users.
Specifically, the login cookie seems to have gotten corrupt. Maybe Google changed its design. So when you visit the bookmarks URL (
https://www.google.com/bookmarks/), you find that nothing appears. The browser enters a redirect loop - bookmarks forwarding to the login page forwarding back to bookmarks. This also causes GMarks to fail to load anything.
The workaround (don't know if it will be a permanent fix or not) is to log out and back in from some other Google service. Go to your favorite Google page (like search -
https://www.google.com/, or mail -
https://mail.google.com/), click on your picture in the upper-right corner and then click "Sign Out". Then sign in again (signing out from Mail will automatically redirect you to a sign-in page. The search page will have a "Sign in" button in the upper-right corner).
Then go back to the bookmarks page - it should display OK. You can now refresh GMarks and it will pull your bookmarks properly.