Single-column row with ID horizontal-gal and this bit of CSS:

#horizontal-gal .et_pb_column {
    overflow-x:auto;
    white-space:nowrap;
}

#horizontal-gal .et_pb_image {
    display:inline-block;
    margin-right:40px;
    height:300px;
}

#horizontal-gal .et_pb_image_wrap,
#horizontal-gal .et_pb_image_wrap img {
    height:100%;
    width:auto;
}