Gillean is an abomination!
Even Jilly-Ann would be preferable.
I prefer
Gillian.
Jillian is etymologically correct, but perhaps it's just too close to
Julian for comfort? Or maybe I'm just set in my ways. Anyway, I've seen many
Gillian people and only one
Jillian so the majority seems to concur.
The only problem is
Gill! I instinctively pronounce it like the fish's body part or the liquid measure, but using
Jill as a nn for
Gillian is counter-intuitive. My solution, inside my head, is to keep
Gillian as a mn. Which I gladly would: I like it a lot.