If you haven’t noticed already, Google’s Gmail doesn’t seem to allow CSS in your email. Yep, this includes basic stuff, like font-family. We’ve recommended against using CSS too much in your HTML email anyway, but for the most part, most of us have gotten by with it (especially for simple stuff like font-family). I’m sure Google will take care of this later, especially since Gmail is technically still in "Beta". But for now, you might want to view your email in Gmail just to make sure it still looks ok.