In short, no.
Email clients very rarely support HTML5 or CSS3, and so it will break in any clients that don't.
You could always have a fallback, but the load time won't change too much, and adding CSS3 simply adds more code to keep track of.
I would recommend using image tags and hosting the images on your server, and that's if you really have to use gradients at all.
With email, generally the simpler the better. Personally, I prefer recieving plain text.
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