This article explains how to select all Gmail emails at once from the desktop website. This will let you apply a new label to the selected messages, delete emails in bulk, mark them as read, and more.
How to delete all or multiple emails in Gmail at once?
Once you've found the emails you want to manage, you can mark them all in two easy steps.
Click the label containing the emails you want to select. For example, to select all the emails in your Gmail inbox, click Inbox.
Click the checkbox directly below the search bar and above your emails to select all messages currently displayed. Or, select the arrow next to this checkbox to select the types of emails to select, such as Read, Unread, Starred, or Unstarred.
