each gallery type is in a sub folder under the galleries/ folder, and is called by using a link to the i de3x.php page for that gallery type, for example, this code will gallery 1 into the /galleries/ribbon_fullsize/ gallery type:
$pg_id = '1';
$pg_theme = 'dark';
you could make a copy of the /galleries/ribbon_fullsize/ gallery folder to create individual galleries.
see the "How to customize a PowerGallery design " tutorial in the archived documentation section of the power gallery support page: