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;
}