As with gallery callouts, you first need to build out your gallery as an asset. Be sure to fill in the description or alternative text for screen readers and the caption to be displayed with each photo. After you create your gallery, you just drop the asset in your main content region and it automatically formats it in a gray box with the caption below. 

The gallery spans the full width of the region with arrows on on either side directing the user to click through for more images. If you only have one image in your gallery, the arrows will not be visible.  Your gallery should only include horizontal images that are all the same size, 805 x 537 px.