composer require league/oauth2-google

use League\OAuth2\Client\Provider\Google;
$provider = new Google([
    'clientId'     => '{google-client-id}',
    'clientSecret' => '{google-client-secret}',
    'redirectUri'  => 'https://example.com/callback-url',
    'accessType' => 'offline',
]);
        if (!empty($_GET['error'])) {
            // Got an error; probably user denied access
            exit('Got error: ' . htmlspecialchars($_GET['error'], ENT_QUOTES, 'UTF-8'));
        } elseif (empty($_GET['code'])) {
            // If we don't have an authorization code, then get one
            $authUrl = $provider->getAuthorizationUrl([
                'scope' => [
                    'https://mail.google.com/'
                ]
            ]);
            header('Location: ' . $authUrl);
            exit;
        } else {
            // Try to get an access token (using the authorization code grant)
            $token = $provider->getAccessToken('authorization_code', [
                'code' => $_GET['code']
            ]);
            // Use this to get a new access token if the old one expires
            $refreshToken = $token->getRefreshToken();
        }