Open the right-hand Pages panel and hover over to the + button 

Use the Full page embed to create a new section to the navigation menu that's not a page

Type in the desired name for the page you want to call it.

You can choose to embed a file from Drive, a map, or a video from the Insert sidebar. Additionally, you have the option to write custom code. The page will dynamically fill with the content.

 You can also embed URLs from other websites, web apps, or gadgets, such as Steegle News, to enhance your page with external content.