Click on Tools at the top of Internet Explorer > Click on Internet Options > Click on the Content Tab > Under Personal Information Click on the AutoComplete Tab > Click on Clear Passwords. Try Gmail again and see if that's done it. If so IE should throw up a prompt box offering to remember the password for you after you've, hopefully, had to type it in. Tell it no, no thanks and never again.
Sorry, should have said. If using Firefox it's Tools > Options > Privacy > Passwords > uncheck 'Remember Passwords' box then 'View Saved Passwords' and remove those you don't want there.