Understanding How to Run the Gmail API Using the Service Account Type OAuth Client

I am trying to set up a forwarding email address for users in Google Apps tenants by referring to the page below, but the API fails to respond.
developers.google.com/gmail/api/v1/reference/users/settings/forwardingAddresses

I have tried the following, but could you tell me if there are any wrong or missing steps?Thank you for your cooperation.

1. Create OAuth Client

Accessed the API Manager in the Google Developer Console and created an OAuth 2.0 client of type サービスService Account Client 」.Check "Enable Google Apps Domain-Wide Delegation" for created clients.

2. Granting authority

"Google Apps Management Screen
　　->Security->Advanced Settings
　　- From > Manage API Client Access,
　Grant https://mail.google.com/" permissions to the created client

3. Get Access Tokens

Access tokens are obtained using the PHP library published on the page below.
developers.google.com/api-client-library/php/start/installation

The PHP codes for obtaining access tokens are as follows:

<?php

require_once realpath(dirname(__FILE__).'/Google-api-php-client/src/Google/autoload.php');

$client_email='[email protected]';
$private_key=file_get_contents('MyProject.p12');

$scopes=array("https://mail.google.com/");

$credentials = new Google_Auth_AssertionCredentials(
    $client_email,
    $scopes,
    $private_key
);

$client=newGoogle_Client();

$client->setAssertionCredentials ($credentials);
if($client->getAuth()->isAccessTokenExpired()){
  $client->getAuth()->refreshTokenWithAssertion();
}

var_dump($client->getAccessToken());

?>

4. Run Gmail API

URL: https://www.googleapis.com/gmail/v1/users/ email address/settings/forwardingAddresses
　methods:GET
　headers:
　　Authorization:Bearer Access Token

→ The response to this request is the following error response:

Response Status: 400
　Response Body:

{
"error": {
    "errors": [
        {
            "domain": "global",
            "reason": "failedPrecondition",
            "message": "Bad Request"
        }
    ],
    "code"—400,
    "message": "Bad Request"
}