Gmail and other Web Browser-based email clients do not support <style>
tags in the head section of the document (or a <head>
section at all, for that matter) as they conflict with their page's CSS. You will need to move/duplicate your CSS inline as well.
e.g.:
<head>
<style>
a { color: #ff0000; }
</style>
</head>
<body>
<a href="#"></a>
</body>
becomes...
<body>
<a href="#" style="color:#ff0000;"></a>
</body>
Some online tools can aid you with this, but they only support basic CSS. Chances are you'll need to do it manually, and remember to only style inline in the future.
Try http://premailer.dialect.ca/ or http://zurb.com/ink/inliner.php