<?php $loop = new WP_Query( array( 'post_type' => 'gallery',
'posts_per_page' => 100 )
);
while ( $loop->have_posts() ) : $loop->the_post();
if ( $gallery = get_post_gallery( get_the_ID(), false ) ) :
$img_ids = explode( ',', $gallery['ids'] );
/* Loop through all the image and output them one by one */
foreach( $gallery['src'] as $key => $src ) : ?>
<img src="https://wordpress.stackexchange.com/questions/307061/<?php echo $src; ?>" class="my-custom-class" alt="Gallery image" />
<?php
$image_post = get_post( $img_ids[ $key ] ); ?>
<p class="wp-caption-text"><?php echo $image_post->post_excerpt; ?></p>
<?php endforeach;
endif;
endwhile; wp_reset_postdata(); ?>
