Custom template should be archive-gallery.php instead of archive-posttype.php
It's archive-$posttype.php where $posttype is the custom post type slug.
WordPress first looks for archive-$posttype.php and if it's not available then chooses archive.php
Update
Same for the custom taxonomies also , the flow is:
taxonomy-$taxonomy-$term.php --> taxonomy-$taxonomy.php --> taxonomy.php --> archive.php
So for gallery_categories custom taxonomy use taxonomy-gallery_categories.php