Use a gallery widget to insert a previously created gallery into the layout of one of your templates. In “Customizer > “Layout” you insert them as shown below.Drag and drop the gallery widget into any other column. Click on it to change the gallery or gallery style. Delete by clicking the trash can icon.
The gallery will fill with available content width, which changes based on browser window (viewport) size. So make sure the images in your gallery are large enough to accommodate the most common window sizes. If you want a full width gallery, make sure there is no padding on the containing block, row or column layout items.