Retrieving a message

retrieve.php

<?php declare(strict_types = 1);

// assuming Google_Client is already configured, and assigned to $client
// $currentUser is the currently authenticated user

$gmail = new Google_Service_Gmail($client);
$googleUserId = $currentUser->getGoogleId();

$response = $gmail->users_messages->listUsersMessages(
  $googleUserId,
  [
    'q' => 'your gmail search query',
  ]
);

$rawData = [];

foreach ($response as $messageResponse) {
  $rawData[$messageResponse->getId()] = $gmail->users_messages->get(
    $googleUserId,
    $messageResponse->getId(),
    [
      'format' => 'raw',
    ]
  );
}