Why is Gabriel a lady in Constantine?

The portrayal of the angel Gabriel as androgynous is not uncommon. See Wikipedia

Angels are described as pure spirits. The lack of a defined form allows artists wide latitude in depicting them. Amelia R. Brown draws comparisons in Byzantine iconography between portrayals of angels and the conventions used to depict court eunuchs. Mainly from the Caucasus, they tended to have light eyes, hair, and skin; and those "castrated in childhood developed a distinctive skeletal structure, lacked full masculine musculature, body hair and beards,....” As officials, they would wear a white tunic decorated with gold. Brown suggests that "Byzantine artists drew, consciously or not, on this iconography of the court eunuch". Some recent popular works on angels consider Gabriel to be female or androgynous.

Tilda Swinton had played an androgynous character previously (in Orlando (1992)), which perhaps explains why the director chose her to play the role. According to the following interviews with Swinton, it was director Francis Lawrence's idea to make Gabriel androgynous.

http://www.phase9.tv/moviefeatures/constantineq&a-tildaswinton.shtml

Gabriel is portrayed as rather androgynous. Would you say Gabriel is more of a he or a she?

Well the angel Gabriel, as the Book of Daniel said, is in the shape of a man. It doesn't say he is a man, just says looks like one and I quite often get given a male frisk when I go through the X-ray at airports so I was up for that. And anyway it's not my idea that I'm impersonating Gabriel, it's entirely Francis Lawrence's so you'll have to blame him for that.

http://www.nytimes.com/2005/02/06/movies/06devr.html

Q. You've played a number of men in your career, most notably in "Orlando." What does the idea of androgyny bring to your performance of Gabriel?

A. The director, Francis Lawrence, did the sensible thing by just presenting the idea to me and then letting it percolate through my brain. It felt like a little bit of a dangerous idea in a big Hollywood movie, and then I ended up thinking it was a good idea, because an angel is not immutable but fluid, and so his identity is amorphous and not strictly human.

Q. How much do you think your looks have to do with this protean quality you have?

A. Quite a lot. The other day, I was going through the airport security and I was searched by a male security guard. I'm very often referred to as "Sir" in elevators and such. I think it has to do with being this tall and not wearing much lipstick. I think people just can't imagine I'd be a woman if I look like this.

In Constantine she is portrayed as sexless person, to represent androgynous in angels. Even confirmed over Wikipedia.

Note: Angels in the Bible are mostly depicted as male (as he is). But whether they are male or female is never made clear and few believers think of them as sexless. And there are various debates being held on their sex again and again, few nice article are here (1, 2, 3).

How do you know what angels look like?

I think the point of casting Tilda Swinton is because nobody can say with certainty that angels are considered male or female. Yes I know the scriptures portray them in certain ways, but nobody knows for certain. You could also argue why the director didn't cast a naked baby with tiny wings and curl hair as Gabriel. Everything is up to the viewers'/believers' interpretation, like many other things about religion. The fact of the matter is that nobody knows for certain what angels are considered to be and that's why I think the director cast a woman to play Gabriel.

I think this is one (of many) things that make Constantine a great movie.