The template deliberately only changes headings when you change the font in template manager because this can have a dramatic effect on the performance of your site if you use it everywhere. Nevertheless, if you still want to do it, Use this code...
body {
font-family: 'Zeyada', 'Helvetica', arial, serif;
}
This is how to create a custom CSS compatible with Gantry 4...
http://docs.gantry.org/gantry4/tutorials/adding-a-custom-stylesheet
Regards, Mark.