My source says Powers plays fast and loose with genres, another manga trait. I didn't notice it that much in tAG, which seemed a merry old romp most of the time with lots of this'n'that thrown in for fun. Err- mangaish. As is the lack of explanation of just what opening the Anubis gate was supposed to accomplish in the first place.
I was also wondering just *why* it's considered steampunk, given the absence of steam in the thing. Think of it as a graphic novel, though, and it starts looking very much like The League of Extraordinary Gentlemen.