GMail is one of the few common email clients which does not support stylesheets.
It is now possible to use the Mustache function fulcrm.post.premailer to convert a piece of HTML such that all styles are inlined.
{{#fulcrm.post.premailer}} <!DOCTYPE html> <html> <head> <style type="text/css"> /* your CSS to be inlined here */ </style> <style type="text/css" data-premailer="ignore"> /* will be left in the <head> */ </style> </head> <body> <!-- your usual HTML here --> </body> </html> {{/fulcrm.post.premailer}}