There are only 2 troyans known for mac. They only install if you do install them yourself. They were distributed on p0rn sites, and pretending to be video drivers you had to install to see the movies.
They change your DNS settings then display advert when browsing the new, but do not send emails like you say.
One thing you have to know is that anyone can send an email that seems to be sent from you, without being sent from your computer. Even without your account details, it can be a friend of yours who has an infected PC who is sending this emails that seem to be coming from you.
There is also the possibility that your Gmail account has been hacked as well. I'd simply recommend you change your gmail password and choose a complex one.
p.