Once you press the “send” button in gmail, it’s gone and it’s out of your control. You can’t do anything about it. Either you can press “undo” as quickly as you can or ask the relevant person or party to delete it. What if you lose both the chances? Though we ask a relevant party to delete, there is only a 50% chance that it will get deleted, maybe it is deleted after taking a copy, so you are actually helpless at this moment.
After typing the email & attaching the documents, you can click on the clock icon (turn confidential mode on/off) which is found next to insert photo icon which is in the latter part of the composing window. After clicking on the clock icon you can select the time according to your preferences, you can select, by when your email should get disappeared.