Well, first of all, I would point out that Gin's outfit is not very similar to a cape design. It actually tapers in at the waist, which is where you are going to have the most difficulty.
Nearly everyone I've known who has made the outfit has put a zipper up the middle there as well.
You can go with a rounded, dress like form (with a triangle cut out of the front) for the area from his waist down, or you can simply go with a straight design. So far I've seen both done adequetly.
I would suggest you take a pattern closer to this http://www.mccallpattern.com/item/M5026.htm
and modify it (this one would require it being longer)
I wouldn't really suggest using that exact pattern, just one that's closer to that general form.