Open Gmail in a web browser
Sign in to your account
Go to the folder you want to clear, such as Inbox or All Mail
Click the checkbox at the top left to select all visible emails
Click the message that appears to select all conversations in that folder
Click the Delete icon
Repeat for other folders if needed
To delete all Gmail mail, go to All Mail and repeat the same steps
Empty the Trash folder
Go to Trash
Click Empty Trash now
Confirm the action