It was in 2009 when Google originally introduced the ability to undo the sending of an email within Gmail. The feature, added as an option as part of Google Labs, had to be explicitly enabled; in 2015, it was rolled out to all Gmail users – but only on the web.
Google added the feature to Gmail’s iOS app in 2016 but didn’t quite get around doing the same for Android. That changes today, as noted by Android Police and Google"s support documentation, as of version 8.7, Gmail on Android supports the ability to undo sent emails.
The feature has been available in Google’s Inbox app for both Android and iOS, and the implementation in the Gmail app is pretty much identical to that. When sending an email, the app first prompts users with an option to cancel that process, and once the email is “sent,” it offers users with the option to undo that.
It does appear to also require a server-side update, however, meaning that it may not show up even if you are on the right version. It’s worth trying a restart of the Gmail app if that happens.