If you've been waiting for Google to bring the 'Undo Send' feature to Gmail, well, you're in luck, as that feature is now available on Gmail for Android. Do keep in mind that you will need your Gmail app to be updated to version 8.7 (at least) in order for this feature to work. The 'Undo Send' feature will basically give you a few extra seconds to prevent an e-mail from being sent after you hit that 'Send' button, just in case you notice something needs to be fixed at the last second, which is something that probably happened to most of you at least once.
Interestingly enough, Gmail v8.7 landed last month, but this feature was not present straight away, so it seems like Google did some changes on the server-side, or something of the sort, as it is now present in app versions from v8.7 and up. So, how does this work exactly? Well, when you write up and e-mail and hit that 'send' button, a notification will pop up at the bottom of the display, saying that the e-mail has been sent. That's basically everything that has been happening before this feature arrived, but now, you also get the 'undo' button on the right of that notification, and for a few seconds, you can click it. If you do, the e-mail will not be sent, and you will be returned to the drafting screen.
This feature seems to be wonky on some devices still, though, as it does not show up on some phones for some reason, but if you force close the Gmail app, and you're on v8.7 or up, you should be able to see it and use it. In any case, this feature has been a long time coming, and quite probably one of the most requested features for Gmail for years now. Google has even included the mention for 'undo send' feature in its support documents, in case you're interested in reading that.