Some questions....
Do you have gmail as a separate account in Mail? How do you have gmail set in Mail pop or imap?
Do you want to just delete specific emails, or the account in full?
If you have the emails showing in the inbox of Mail already, just hold down the shift key while you click on the emails then hit the trash can, essentially trashing bulk.
It might be easier for you to log into gmail’s web and delete from there - if you still have them on the server.