This article explains how to quickly delete emails in the official Gmail app for Android. You'll learn how to send multiple emails to trash at once and how to set up a swipe gesture to delete individual emails in seconds.
How to Quickly Delete All Gmail Emails on Your Phone! (Android & iPhone)
If you want to delete many emails at once or if you want to delete all emails in a folder, follow these steps:
Tap the three-line menu at the top of the app and choose the folder you want to delete the emails from.
Tap the icon to the left of one of the messages you want to delete, then tap the second message, and so on until you've made your selection. Or, if it's easier, tap and hold the subject or preview to select an email.