Woocommerce custom field on emails outputs ‘Array’

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;
}