I love both
Gemma and
Jenna, although I slightly, but only slightly, prefer
Jemma. Have you thought about
Jemima? Thats another similar one.
As for
Gemma vs.
Emma, I prefer
Gemma, both because it's less popular (especially if you are in the U.S-
Gemma doesn't even crack the top 1000!) and because of the sound. I also love the nn Gem.
I also prefer
Jenna to
Emma- something in
Jenna really appeals to me, I'd probably have it in my Top 10 if I hadn't already been beaten to using it :( The only qualms I have about this is its similarity to your own name- this would bug me, but it might not bug you. If it was me, I'd go for something else- but it's nto as if it would be siblings named
Jenny and
Jenna.