Remove css gallery

Add this into functions.php

add_filter('gallery_style', create_function('$a', 'return "
<div class=\'gallery\'>";'));

also, add this is your themes css:

<style type='text/css'>
        .gallery {
                margin: auto;
        }
        .gallery-item {
                float: left;
                margin-top: 10px;
                text-align: center;
                width: 33%;
        }
        .gallery img {
                border: 2px solid #cfcfcf;
        }
        .gallery-caption {
                margin-left: 0;
        }
</style>

per:
https://sivel.net/2009/02/remove-gallery-style-tags-one-liner/