The use of fonts is currently limited by support in e-mail clients. We advise using web-safe fonts to ensure your signatures are displayed the same way in different e-mail clients.
Also, Gmail doesn't support methods for importing or linking fonts from external sources; it strips header styles, and there's no way to inline custom font declarations. Email has so many more constraints than the web because of the many and different ways the HTML is consumed and altered by clients.
You'll need to inline your CSS and use font stacks that fall back to acceptable system fonts.