...I am in no way an experienced artist when it comes to traditional mediums. I was, however forced to work a lot with gouache in college (damn you Bruce Claypool!) Every artist has their personal opinion on the matter of course, and I wouldn't say that mine is right, per se. But I can throw up a pretty convincing argument that gouache is THE most difficult of the traditional painting mediums.
Many artists have mastered it in ways that boggles my mind. Some swear by the medium. But I found it incredibly difficult to work with. You can't paint over it without having the bottom colors seep through, which means you can add color, but never truly correct. If you touch the dried paint you'll forever have a thumb print on it, and if you glare at the paint it seems to increase it's desire to thwart you.
Gouache, suffice it to say is Satan's paint.
Richard would no better for sure, but looking at the larger picture, I'd guess it was MOSTLY gouache which leaves me pretty damn speechless at the technique. Not a hint of muddiness anywhere and very, very crisp details. Stunning.