Use the attached file with a .htaccess file with the following:
RewriteRule ^galleries/(.*)$ gallery-redirect.php?user=$1 [L]
Now, if a user visits www.domain.com/galleries/username
it will redirect them to that users gallery.
It's in use at my site if you try www.furcartzone.com/art/Turquoise
The attached file is a modified version of one that was used to redirect profiles.