Add a new page in the WordPress backend and name it something like Gallery. Next, change the page template to Gallery Page and click Publish. After publishing, you should see a Gallery Settings box appear underneath the main content box that allows you to select the image aspect ratio.
The gallery page is used to display your images that open up in a lightbox when clicked. This differs from the portfolio page as the portfolio posts contain a lot more information regarding the work displayed.
Gallery pages also contain infinite scrolling!
To add a gallery to the gallery page, simply upload all your images using the media upload button inside the page. The image title will be displayed underneath the image when opened up in the lightbox.