You can try with:

function gfm_order_meta_keys( $keys ) {
    global $post;
    $new_keys = array('gfm_pickup_date'=>get_post_meta($post->ID, 'gfm_pickup_date', true));
    
    $keys = array_merge($keys, $new_keys);

    return $keys;
}

You can try with:

function gfm_order_meta_keys( $keys ) {
   
    $new_keys = array('Pickup date'=>'gfm_pickup_date');
    // if the action is used by another plugin, we merge keys into one array
    $keys = array_merge($keys, $new_keys);

    return $keys;
}

You can try with:

function gfm_order_meta_keys( $keys ) {
    global $post;
    $keys['Pick-up date'] = get_post_meta($post->ID, 'gfm_pickup_date', true);
    return $keys;
}

You can try with:

function gfm_order_meta_keys( $keys ) {
    global $post;
    $new_keys = array('gfm_pickup_date'=>get_post_meta($post->ID, 'gfm_pickup_date', true));
    
    $keys = array_merge($keys, $new_keys); 

    return $keys;
}