GUI~ has a graphical user interface. When applying for a design patent, you must apply together with the product. In other words, if your interface is used in a mobile phone, you must display the interface in the mobile phone and apply together with the mobile phone. You can't apply for GUI interface graphics separately.
As for the submitted materials, there are at least three documents: design request, design picture or photo, and brief description of design. I suggest you apply to an intellectual property agency.