(defun setup-gui-cm-binding ()
  "Set up C-m to toggle menu bar in GUI Emacs without affecting RET."
  (when (display-graphic-p)
    (define-key input-decode-map [?\C-m] [C-m])
    (global-set-key (kbd "<C-m>") 'menu-bar-mode)))

(setup-gui-cm-binding)