// WP_Query arguments
$args = array (
    'post_type'              => array( 'gym' ),
    'order'                  => 'ASC',
    'orderby'                => 'date',
    'meta_query'             => array(
        array(
            'key'       => 'cast',
            'value'     => 'Boss - test',
            'compare'   => 'LIKE',
        ),
    ),
);

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

// The Loop
if ( $query->have_posts() ) {
    while ( $query->have_posts() ) {
        $query->the_post();
        echo the_title();
    }
} else {
    // no posts found
}

// Restore original Post Data
wp_reset_postdata();