Because this repo contains submodules, clone it using
git clone --recursive git://github.com/kbanman/fuel-GasCap.git

or install using Fuel's Oil utility: php oil package install gascap

Then in app/config/config.php, add gascap to the auto-loaded packages list:
'packages' => array( 'auth', 'gascap', ),

And finally in app/config/auth.php, set the driver parameter to be GasCap.

Use the following SQL to create the necessary table:

CREATE TABLE `users_gascap` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `created_at` int(11) unsigned DEFAULT NULL,
  `updated_at` int(11) unsigned DEFAULT NULL,
  `group` int(11) NOT NULL DEFAULT '1',
  `email` varchar(128) NOT NULL,
  `last_login` varchar(25) DEFAULT NULL,
  `login_hash` varchar(255) DEFAULT NULL,
  `oauth_token` varchar(255) DEFAULT NULL,
  `oauth_token_secret` varchar(255) DEFAULT NULL,
  `identity` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `email` (`email`),
  KEY `identity` (`identity`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

Has only been tested with Google as the provider, but should work for any OpenID 2.0 provider. The example code is currently modelled so that users must be "invited" to create an account;
An activation code is generated and attached to a group, and the user can activate that account with any supported OpenID identity.

This is all still in a very alpha state; I'll be adding better example code and updates as I have time.