gistfile1.php

                <?php
                // WP_Query arguments
                $args = array (
                    'posts_per_page'         => '6',
                    'post_type'              => 'attachment',
                    'meta_query'             => array(
                        array(
                            'key'       => 'galeriaimg',
                            'value'     => 'sim',
                        ),
                    ),
                );

                // The Query
                $attachment = new WP_Query( $args );

                // The Loop
                if ( $attachment->have_posts() ) {
                    while ( $attachment->have_posts() ) {
                        $img_url = wp_get_attachment_url( $attachment->ID, 'medium', false );
                        $img_url_full = wp_get_attachment_url( $attachment->ID, 'large', false );
                        echo '<a class="fancybox" href="'.$img_url_full.'">';
                        echo '<img src="'.$img_url.'">';
                        echo '</a>';
                    }
                } else {
                    echo '<h1>ERRO1</h1>';
                }

                // Restore original Post Data
                wp_reset_postdata();
                ?>