"...Gmail has not historically supported classes or id's in the <head> of an email, which forced email designers to use inline CSS to style their emails. Now, Gmail will support embedded CSS with classes and id's, which removes the need for inline CSS in Gmail. This also means Gmail will finally support media queries--and responsive email...."
This is huge for folks who specialize in email marketing.
is a list of everything that are supported...
and if you'd like some free responsive templates, consider:
28 Free Responsive Email Templates | Practical Ecommerce