// This is the meta query

$query = new WP_Query([
    'meta_query' => [
        'relation' => 'OR',
        ['key' => '_glsr_ranking', 'compare' => 'NOT EXISTS'], // this comes first!
        ['key' => '_glsr_ranking', 'compare' => 'EXISTS'],
    ],
    'order' => 'DESC',
    'orderby' => 'meta_value_num',
    'post_status' => 'publish',
    'post_type' => 'page', // change this as needed
    'posts_per_page' => 10, // change this as needed
]);

// This is the meta query SQL result (notice the OR)

SELECT SQL_CALC_FOUND_ROWS wp_posts.ID
FROM wp_posts
LEFT JOIN wp_postmeta ON (wp_posts.ID = wp_postmeta.post_id AND wp_postmeta.meta_key = '_glsr_ranking' )
LEFT JOIN wp_postmeta AS mt1 ON ( wp_posts.ID = mt1.post_id )
WHERE 1=1
AND ( wp_postmeta.post_id IS NULL OR mt1.meta_key = '_glsr_ranking' )
AND wp_posts.post_type = 'page'
AND ((wp_posts.post_status = 'publish'))
GROUP BY wp_posts.ID
ORDER BY wp_postmeta.meta_value+0 DESC
LIMIT 0, 10