You can use a plugin that gives you php code and put in in ATO>Add HTML/CSS Inserts>HTML Inserts: Body Top.
I tried this with NextGen Gallery and used the following php code where the three numbers are for gallery ID, Width, and Height.
HTML Code:
<?php echo nggShowSlideshow(1,900,120); ?>
You would have to determine the ID for your particular gallery and then add something like the following to the CSS Inserts in the same section so it doesn't push the layout down and is behind it.
HTML Code:
#so1_1 {
z-index: -1;
position: absolute;
}