To delete all emails in Gmail, follow these steps:

1. Log in to your Gmail account on a web browser.
2. On the left sidebar, click "More," then select "All Mail" to view all your emails.
3. Click the checkbox at the top left to select all the emails on the current page.
4. Click the blue text link that appears, which says "Select all X conversations in All Mail," to select every email.
5. Click the trash can icon (Delete) to move all selected emails to the Trash.
6. To permanently delete them and free up space, go to the Trash folder and choose "Empty Trash now" (emails in Trash are auto-deleted after 30 days).

Note: This action permanently deletes all your emails, so be sure to save any important messages before deleting.