The data is already in the library, so this is all it takes:

/**
 * API function to return the continent code for a given IP. Defaults to using the
 * current user's IP if not specified.
 *
 * @return string
 */
function geoip_continent_code($ip = NULL) {
  $ip = $ip ? $ip : geoip_ip_address();
  $gi = geoip_instance();

  if (!$gi) {
    return FALSE;
  }

  $id = geoip_country_id_by_addr($gi, $ip);
  $continent = isset($gi->GEOIP_CONTINENT_CODES[$id]) ? $gi->GEOIP_CONTINENT_CODES[$id] : '';

  geoip_close($gi);

  if (variable_get('geoip_debug', FALSE) && !empty($_GET['geoip_debug'])) {
    drupal_set_message(t('GeoIP reports continent: %continent', array('%continent' => $continent)));
  }

  return $continent;
}