If you have an existing Glade project you can Open it through this menu item, or through the Open button of the
toolbar, or through the F3 shortcut, or else on the command line if you start glade from a gnome-terminal. Via the menu, button, or keyboard shortcut you will be presented with a File Selection Dialog to choose the Glade project file.
Like all Gnome (actually all Gtk+) applications tab-completion is active in the File Selection Dialog. Thus you can type Pr<tab> and it will be completed to Project/gwords if that is the unique completion. For non-unique completions you will see a list of possible completions in the File list.