Menus in Gnome applications work in much the same way as other applications. The left mouse button is used to select a menu. You can either click the mouse button on the menu or else hold the mouse button down over the menu. In both situations the menu will stay until you make a choice or else click somewhere else on the window.
Generally there are four types of menu items. The first have a right pointing arrow and lead to further sub-menus. The second type may have a small icon and some text followed by three dots. These items lead to dialogues that ask for further information. The third type also may have a small icon followed by some text but no dots. These items perform their action immediately on selection. Finally, some menu items represent options that can be turned on and off.
All Gnome applications have at least a File menu and a Help menu. The File menu contains at least an Exit with keyboard shortcut to Ctl+Q. The Help menu contains an About item which opens a dialogue that identifies the application, author, version and date.