Note that services and tools for sending emails may be able to inline your CSS for you, allowing CSS in <style>
tags to work in Gmail.
For instance, if you're sending emails with MailChimp, your CSS from <style>
tags will get inlined automatically by default. With Mandrill, you can enable this functionality (although it's disabled by default for performance reasons) by checking the "Inline CSS Styles in HTML Emails" box in the "Sending Defaults" section of the Settings tab: