Firstly, change your gmail password. It's probably gmail as this seems to be a common gmail issue. Secondly, is your browser updated?You probably have auto-update already as a default option on your browser - are you using chrome? hirdly, what date were they sent?
I have gmail but don't use chrome and I have none of these in my sent items.
It might be wise to change all your important passwords just in case. You should really change them once a month anyway.
I'd not worry too much - if it was a known and common attack, you'd have heard about it by now. Google is such a big company, it would have hit the national news.