config() is deprecated and should be replaced by \Drupal::config()

When you deal with the new config system (see https://drupal.org/node/1500260 for more information), you should access properties with \Drupal::config(), rather then config(), which is deprecated.

Before:

$gzip = config('system.performance')->get('response.gzip');

After:

$gzip = \Drupal::config('system.performance')->get('response.gzip');

Note also that in a class you should inject the 'config.factory' service and get the config object from there

class {
  public function __construct(ConfigFactory $config_factory) {
    $this->config = $config_factory->get('system.performance');
  }

  public function other() {
    $gzip = $this->config->get('response.gzip');
  }
}