...
Return to the KiCad main window and open the Gerber Viewer using its corresponding button. Alternatively, you can use the Ctrl-G hotkey. Once the application opens, you can load up gerbers and drill files for viewing by clicking on the 
Open Gerber plot file(s) on the current layer button on the top toolbar, and similar for drill files by clicking on the Open Excellon drill file(s) on the current layer. When browsing, you can shift- or ctrl-click multiple gerber or drill files to open them all at once. Note that the order in which you load the gerbers files changes the order of layers that are shown. Layers that are loaded first will be shown above layers that are loaded later. Toggle the visibility of the layers using the Layers Manager on the right to more easily inspect each of the gerber files. If you also loaded up the drill file, you will notice it as a multitude of letter labellings, known as DCodes, along with the usual colored layer. You can disable the DCodes at any time by clicking on the Show DCode number button on the left toolbar.
...