Gillian has been the default spelling since the Middle Ages, no doubt because people then thought it looked better! And it's not as if gin and giblets are confusing people because they don't start with a j.
I like
Gillian enough to use, and
Jillian not at all. I do enjoy
Jill as an independent name, but not enough to use it.And I was at school with, I think, three
Gillian people and one
Jillian, and nobody turned a hair.