According to the manual
| In the simplest case, SEPARATOR-TYPE consists of only dashes. That
| specifies the default kind of separator.
a string of consisting of only hyphens should give a menu separator
line. This works with the default Xaw3d build, but not in the GTK
version.
To reproduce evaluate:
(define-key-after menu-bar-help-menu [test] '("----") 'emacs-faq)
In the default build this results in a proper separator in the "Help"
menu. But in the GTK build it's shown as a string of four hyphens.