I opened single.php and made a php if statement.
Something like this (not real code, because I removed it again).
<?php if ( is_category(4)) echo {("[gallery columns="6" orderby="post_name"]'); } ?>
category 4 is “Gallery”.
So it means that if a post is in the category gallery it should automatically show all thumbnails instead that I have to manually insert the gallery in the post.
The IF works, but the only problem is that the output are not the gallery thumbnails, but the plain text tags -> [gallery columns="6" orderby="post_name"]