gio:menu-insert-submenu (menu pos label submenu)

menu -- a g:menu object
pos -- an integer with the position at which to insert submenu
label -- a string with the section label, or nil
submenu -- a g:menu-model object with the items of the submenu

Convenience function for inserting a submenu into the menu. Combine the g:menu-item-new-submenu and g:menu-insert-item functions for a more flexible alternative.