Re: What's your view of the definitive Batman costume?
I think it depends what you mean by definitive: the definitive Batsuit, for me, is the Bronze Age version (Adams, Aparo etc) which had quite a light great body. That was to some extent, however, a product of the colouring tech of the time, and it was almost certainly intended to represent something a bit darker.
I usually say "graphite" is the way to go, which I suppose is not far off from "gunmetal". That's with the caveat that I prefer a hint of blue in the cape and cowl, which contrasts better with dark grey.