The method you need does not exist yet? You can access any GitHub route by using the "get" and "post" methods. For example:
$client = new Github\Client();
$response = $client->getHttpClient()->get('repos/KnpLabs/php-github-api');
$repo = Github\HttpClient\Message\ResponseMediator::getContent($response);
Returns an array describing the "php-github-api" repository.
See all GitHub API routes: http://developer.github.com/