g-menu-item-set-submenu (item submenu)

item -- a g-menu-item object
submenu -- a g-menu-model object, or nil

Sets or unsets the "submenu" link of the menu item to submenu.

If submenu is non-nil, it is linked to. If it is nil then the link is unset.

The effect of having one menu appear as a submenu of another is exactly as it sounds.