This may be redundant, and if there is a thread out there already, please delete this one, but I have been unable to find much on the way of CC for gel:email.
When I try, it says CC is not supported. I checked out the jakarta website and it does look like there is CC support for Jelly, but I'm assuming the default library that comes with Clarity does not include that.
I did find this thread with no answer to what sounds like the same question:
How does one enable cc capability for gel:email?