Open Google Account settings: https://myaccount.google.com/
Sign in to the Google account you want to remove
On the left menu, select Data & privacy
Scroll to Delete a Google service or your account
Select Delete a Google service
Choose Gmail
Enter your password if prompted
Follow the prompts to confirm deletion
Review what will be deleted and any alternatives offered
Confirm deletion to permanently remove the Gmail address and its content
If you want to delete the entire Google account instead, go to the same page and select Delete your account