Open the Gmail app on your iPhone
Tap your profile picture in the top right
Tap Manage accounts on this device
Tap Remove from this device
Select the Gmail account you want to delete
Tap Remove from this device again to confirm
If you want to delete the Gmail account from Google permanently, go to myaccount.google.com in a browser
Sign in to the account you want to delete
Tap Data & privacy
Scroll to More options
Tap Delete your Google Account
Follow the on-screen steps to confirm deletion