gio:menu-item-set-section (item section)

item -- a g:menu-item object
section -- a g:menu-model, or nil

Sets or unsets the "section" link of the menu item to section. The effect of having one menu appear as a section of another is exactly as it sounds: the items from section become a direct part of the menu that item is added to. See the g:menu-item-new-section function for more information about what it means for a menu item to be a section.