Edit in JSFiddle

        @media all and (min-width: 960px) {
            #gallery_layout {
                position: absolute;
                top: 50%;
                left: 50%;
                transform: translate(-50%, -50%);
            }  
        }

        @media all and (max-width: 959px) {
        #gallery_layout {
            width: 100%;
        }
        .gallery_content {
            width: 50%;
        }

        }
        @media all and (max-width: 499px) {
        #gallery_layout {
            width: 100%;
        }
        .gallery_content {
            width: 100%;
        }

        }