Yup, that's what I was told by someone who did 
backgrounds in the American market.  Gouache = 
opaque watercolors, an underappreciated medium!

Some artists DO use regular watercolors, and I 
even have a couple of anime backgrounds that were 
done with watercolors specifically for the 
transparency effect.  But mostly, they're done in 
gouache (but a very good grade, not anything sold 
for school kids!), for the rich opaque quality 
and the rendering of such fine details (which I 
am very jealous of! - A friend of mine who 
teaches drafting in college borrowed some of my 
backgrounds to show his class that drafting 
isn't "dry and worthless" at all!).