Apart from adding a cache to the watchlist G4L also offers a "Create bookmark" menu item.
I assume, that the intended functionality is to add the particular cache to one of the user lists (can only guess, as it does not work for me...).
What I am getting instead is a redirection to a gc.com error page, stating the the supplied GUID does not comply with the expected format. Could it be, that G4L did not pick up one of the recent gc.com API changes?