A method of providing, by an image output apparatus, a graphical user interface (GUI) for providing a service to a user is provided, the method including: displaying a selection menu for determining whether to execute a pre-set function related to a service corresponding to an external apparatus when the external apparatus is connected to the image output apparatus; determining whether to display a function list indicating at least one detail function of the service corresponding to the external apparatus, based on a selection input with respect to the selection menu; and displaying the function list based on a result of the determining.