Checkout this repository to your vendor directory, and setup autoloading for ZendService\OAuth2 pointing to vendor/ZendServiceOAuth2/src/ZendService/OAuth2
$loader = new StandardAutoloader();
$loader->registerNamespace('ZendService\OAuth2', 'vendor/ZendServiceOAuth2/src/ZendService/OAuth2')->register();
Instanciate a new OAuth2 (probably in a controller) passing your client secret, client id, the request object and an optional config file to use. You'll possibly want to set the stage1 config variable scope using setConfigValue.
To use authenticate with Google:
$auth = new OAuth2(
'<Your app client id>',
'<Your app secret>',
$this->getRequest(),
'google'
);
$auth->setConfigValue('stage1', 'scope', array('https://www.googleapis.com/auth/userinfo.profile', 'scope'));
$token = $auth->getToken(true);