upload-gcs.gs

function main() {
  const threads = GmailApp.search('label:gcsエクスポート-未処理');
  for(var i = 0; i < threads.length; i++) {
    const messages = GmailApp.getMessagesForThread(threads[i]);
    /* 各メールから日時、送信元、件名、内容を取り出す*/
    const message = messages[messages.length - 1];
    const attachments = message.getAttachments();
    for(var j = 0; j < attachments.length; j++) {
      if (attachments[0].getName() == "書き出す.zip") {
        // GmailAttachment
        isSuccess = uploadFile_(attachments[0]);
        threads[i].removeLabel(yetLabel);
        threads[i].addLabel(completeLabel);
      }
    }
  }
}

function uploadFile_(gmailAttachment) {
  const jsonKey = JSON.parse(PropertiesService.getScriptProperties().getProperty("SAJsonKey"));
  
  const gsAPP = new GSApp.init(jsonKey.private_key, ['https://www.googleapis.com/auth/devstorage.full_control'], jsonKey.client_email);
  const tokens = gsAPP.addUser(jsonKey.client_email).requestToken().getTokens();
  const token = tokens[jsonKey.client_email].token;
  
  const bucketName = '任意のバケット名';
  const uploadFilepath = 'GCSへのアップロードパス exp: `hoge/fuga.zip`';
  const requestUrl = "https://www.googleapis.com/upload/storage/v1/b/"+bucketName+"/o?uploadType=media&name="+encodeURIComponent(uploadFilepath);
  
  const blob = gmailAttachment.copyBlob();
  const bytes = blob.getBytes();
  
  const fetchOptions = {
    method: "POST",
    contentLength: bytes.length,
    contentType: blob.getContentType(),
    payload: bytes,
    headers:{Authorization:"Bearer "+ token}
  };
  
  UrlFetchApp.fetch(requestUrl, fetchOptions);
}