Only do this if you only have one Gmail Account.
Remove Mail Data and Account Information (Advanced Reset)
This is the most thorough manual reset. It removes the Mail app's local data folders, including all accounts, local mailboxes, and cached messages, forcing Mail to start as if it were opened for the very first time.
Disable All Mail Accounts:
Go to the Apple Menu > System Settings (or System Preferences on older macOS).
Click on Internet Accounts.
Select each email account, and for the mail services listed, toggle off the Mail switch or choose Delete Account (if you want a clean slate).
Quit Mail (Command + Q).
Open Finder.
From the menu bar, select Go > Go to Folder... (Command + Shift + G).
Move the following folders/files to the Trash (or to your Desktop as a backup):
Mail Data Folder: ~/Library/Mail (This folder contains all your local messages, attachments, and the mailbox index. Deleting this will require Mail to re-download all messages from the server.)
Mail Container Folder: ~/Library/Containers/com.apple.mail (This holds the app's sandboxed data and settings.)
Mail Preferences: ~/Library/Preferences/com.apple.mail.plist (If you didn't do Step 2.)
Restart your Mac.
Open the Mail app and re-add your email accounts via System Settings > Internet Accounts.