Get all posts from custom taxonomy

add_action( 'init', 'custom_gallery_post_type' );
function custom_gallery_post_type() {
    register_post_type( 'custom_gallery',
        array(
            'labels' => array(
                'name' => __( 'Gallery', 'custum_gallery' ),
                'singular_name' => __( 'Gallery Item', 'custum_gallery' ),
            ),
            'has_archive' => true,
            'hierarchical' => true,
            'public' => true,
            'rewrite' => array( 'slug' => 'custum_gallery' ),
			'taxonomies' => array('custom_gallery_category'),
			'supports' => array( 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'comments' )
         )
    );
	register_taxonomy('custom_gallery_category', 'custom_gallery',
		array(
			'hierarchical' => true,
			'query_var' => true,
			'rewrite' => true,
            'has_archive' => true,
            'hierarchical' => true,
            'public' => true
			)
		);
}

function customgallery( $atts, $content = null ) {
	$content='<ul class="gallery_menu">';
	$_categories = get_categories('taxonomy=custom_gallery_category');
	foreach ($_categories as $_cat){
		$content=$content.'
			<li class="'.$_cat->name.'" >
				<a href="'.get_term_link( $_cat->slug, 'custom_gallery_category' ).'" title="'.$_cat->name.'">'.$_cat->name.'</a>
			</li>';
		}
	$content.='
		</ul>
		<div style="clear:both;"></div>
		<ul class="gallery_item">'; 

	$_item = new WP_Query(array('post_type' => 'custom_gallery', 'posts_per_page' => '-1'));
		while ($_item->have_posts()) : $_item->the_post();
		$content.='<li><a href="'.get_permalink().'" rel="bookmark" title="'.get_the_title().'"><img src="'.wp_get_attachment_image_src(get_post_thumbnail_id($_item->ID),'thumbnail')[0].'"></a>';
		$content.='<a class="gallery_title" href="'.get_permalink().'" rel="bookmark" title="'.get_the_title().'">'.get_the_title().'</a></li>';

		endwhile;

		$terms = get_terms('custom_gallery_category');

		$content.=$terms->slug;

	return $content.'</ul>'.'<div style="clear:both;"></div>';
}
add_shortcode('customgallery', 'customgallery');

its working good with any theme which supports featured image.
i can get posts for sub-categories on index.php
i can get single posts on single.php
I want to get all taxonomy posts without using taxonomy.php or any external template. So is it possible to get all custom taxonomy posts on index.php?

Example:
wordpress/?custom_gallery_category=gallery-test-1
Gives the result on index.php.
I want to dosomething like that
wordpress/?custom_gallery_category=is there a tag for this like ‘ALL’