Refresh token on Google OAuth2 PHP library - php

Question

I implemented Google OAuth2 for user login on my website.
It works but after 1 hour token expires and login fails.
I read on the web that I need to get the Refresh Token (for Facebook Login I used a Long Lived Token), but code I tried doesn't work.
Here the code:
//LOGIN CALLBACK FROM GOOGLE
$gClient = new Google_Client();
$gClient->setApplicationName(SITE_TITLE);
$gClient->setClientId(get_option('google_api_id')->value);
$gClient->setClientSecret(get_option('google_api_secret')->value);
$gClient->addScope('profile');
$gClient->addScope('email');
$gClient->setRedirectUri(SITE_URL."/login/google/google-callback.php");
if(isset($_GET['code'])) {
$gClient->setAccessType('offline');
$token = $gClient->fetchAccessTokenWithAuthCode($_GET['code']);
$gClient->setAccessToken($token['access_token']);
$_SESSION['google_access_token'] = $token['access_token'];
}
if($gClient->getAccessToken()) {
// Get user profile data from google
$google_oauthV2 = new Google_Service_Oauth2($gClient);
$gpUserProfile = $google_oauthV2->userinfo->get();
}
...
This first snippet works fine.
In this second snipped, when user change page, I verify if login is still active:
$gClient = new Google_Client();
$gClient->setApplicationName(SITE_TITLE);
$gClient->setClientId(get_option('google_api_id')->value);
$gClient->setClientSecret(get_option('google_api_secret')->value);
$gClient->addScope('profile');
$gClient->addScope('email');
$gClient->setAccessType('offline');
$gClient->setAccessToken($_SESSION['google_access_token']);
if($gClient->getAccessToken()) {
if ($gClient->isAccessTokenExpired()) {
$gClient->fetchAccessTokenWithRefreshToken($gClient->getRefreshToken());
}
$google_oauthV2 = new Google_Service_Oauth2($gClient);
$gpUserProfile = $google_oauthV2->userinfo->get();
...
}
This second snipped doesn't work, because method fetchAccessTokenWithRefreshToken($gClient->getRefreshToken()) fails because $gClient->getRefreshToken() is NULL.
I debugged the callback and I saw that $token = $gClient->fetchAccessTokenWithAuthCode returns an array without "refresh_token" field.
Can anyone help me?
Thanks
Bye