Remove the Add Gallery button on post / page edit screen.

envira-gallery-remove-media-button.php

<?php
/**
* Plugin Name: Envira - Remove Add Gallery Button
* Plugin URI: http://enviragallery.com
* Version: 1.0
* Author: Erica Franz
* Author URI: https://fatpony.me
* Description: Removes the Add Gallery button from all Visual Editors in WordPress
*/

add_filter( 'envira_gallery_media_button', 'remove_envira_gallery_media_button' );
function remove_envira_gallery_media_button( $button ) {
    return '';
}

Nicely done! 👍