GUI Design Participants

The "marketing personnel" is there to dictate the functional requirements of the GUI (as opposed to which widgets will be used and how exactly it will look).

The GUI designer is there to take the requirements and turn them into a GUI with proper flow for the requirements. GUI design is a separate profession from programming, and often uses a dedicated person.

The system architect is there to make sure the proposed GUI fits in the application's architecture, or to modify the architecture to fit the proposed GUI.