use Superbalist\Flysystem\GoogleStorage\GoogleStorageAdapter;
use League\Flysystem\Filesystem;
$credentials = new \Google_Auth_AssertionCredentials(
'[your service account]',
[\Google_Service_Storage::DEVSTORAGE_FULL_CONTROL],
file_get_contents('[[path to the p12 key file]]'),
'[[your secret]]'
);
$client = new \Google_Client();
$client->setAssertionCredentials($credentials);
$client->setDeveloperKey('[[your developer key]]');
$service = new \Google_Service_Storage($client);
$adapter = new GoogleStorageAdapter($service, '[[your bucket name]]');
$filesystem = new Filesystem($adapter);