您需要在单独的请求中修改该邮件，然后删除 UNREAD -标签.

You need to modify the message in a separate request, and remove the UNREAD-label.

POST https://www.googleapis.com/gmail/v1/users/me/messages/1533cb4d7dac1633/modify?access_token={ACCESS_TOKEN}

{
 "removeLabelIds": [
  "UNREAD"
 ]
}