Jeffery B. Rancier’s March 12 post on help-gnu-emacs@gnu.org has
this useful LISP snippet for Windows Emacs users:

(defun jbr-w32-simulate-Alt-tap ()

(interactive)
(w32-send-sys-command 61696))
(global-set-key [C-tab] 'jbr-w32-simulate-Alt-tap)

I wonder if there’s a similar way to do that under GTK…