Replacing an Image gallery

Figured it out.

//Remove original Gallery
function remove_the_first_gallery( $output, $attr ){
    $output="<!-- gallery 1 was here -->";   // Must be non-empty.
    return $output;
}

add_filter( 'post_gallery', 'remove_the_first_gallery' );

That removed all galleries on the page. But since my new gallery isn’t technically a post_gallery, it was left alone.