Is it possible to use gradients on elements in newsletter emails, using CSS? Is the full property supported?
Support for CSS gradients has improved nowadays in email clients, something like this will work in the majority:
background-image: linear-gradient(to top, rgba(0, 0, 0, 0), transparent);
Outlook on Windows doesn't support that, but you can use VML (Vector Markup Language) to make it work there too, i.e.:
Text over gradient
I wrote a guide on gradients in email for the Maizzle framework, it also shows how to do body background gradients in Outlook, and a few other tips.