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}}