Gmail cannot send email with <style> tags. As far as I can tell. If you do send one using Gmail, it will automatically inline all styles. This is a problem as some styles cannot be inlined, such as @media queries. Other email clients are capable of sending <style> tags, so Gmail is capable of receiving these, but if you attempt to forward one of these Gmail will strip away the <style> tag and inline everything it can.
This includes the examples given on the Gmail CSS style support page.