I feel like I did once know one, but so far back in the dark mists of my childhood that I can't really definitely say much about it.
I could see how
Gillian could have once been a male name, since
Jillian is a form of
Julian, which used to be unisex/feminine, just like
Christian.
I prefer
Gillian with a hard G, but soft G seems to be the way most pronounce it (and, as it's related to
Jillian, I suppose that makes sense). It's interesting though that
Gillian is most often presumed to be soft-g but names like Gilligan and
Gillis are hard-g.