g-menu-insert-section (menu position label section)

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

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