I agree with Hesh (I guess I kind of have to, since he referenced to me
). I think what you're seeing is braces pulled from a parts bin at random - keeping in mind that it was probably filled with braces mixed together from dozens of different boards, and a variety of cut angles. It looks to me like slab cut spruce, probably run through whatever shaper they used and chosen fairly haphazardly.
Even when you find an apparent pattern like this, I would hesitate to infer a strategy behind it unless you can hypothesize a motive for that pattern. There's no real motive I can imagine for running treble braces at about 7 degrees off flat and bass at around 20 degrees off flat. It looks like a normal results from slab sawn spruce. Even if the bass is consistently off further than the treble, it's important to remember that the odds of drawing a royal flush are exactly the same as drawing any other combination.
I think it would take a great deal of careful documentation of grain angles to gain any statistical certainty that the pattern is significantly more likely to be intentional than random. Statistics often surprise us when the numbers tell us what we though was totally uncanny was actually not unlikely at all.
If there is indeed an uncanny similarity in the degree and direction of the grain slant, there may still be a reason behind it. Since there's no immediately apparent benefit to tone or structure I can think of, I would look first at reasons like supply and manufacturing processes.
Consider this hypothesis:
Gretsch receives their spruce rough, in a wide range of grades with some boards ranging up to 20 degrees off quartered. They have certain minimum criteria in receiving to grade spruce worthy of tops. The other boards are tossed off to become less visible brace stock. Due to the shape of the boards they receive, yield is usually maximized (or labor is minimized) by cutting them flat sawn. Cutting the flat boards in to braces is most easily and consistently done with the cupped side down, leaving the grain in the resulting braces always angled down in the same direction. The boards are habitually tossed in to a box from the same direction relative to the cut, went to the shaper or template pulled out in the same direction, etc.
That's just one quick idea, not really thought out and likely full of holes. I'm sure there are tons of better ideas as to how they could have ended up that way. Maybe they got their top woods in 9"-10" width, and the least quartered end was cut off to become brace stock. Then for some other reason, the majority got cut and shaped with the grain angled in a consistent direction. Who knows?
My point is that unless there is a good hypothesis as to quality-motivated intentions for a seemingly consistent pattern, it's often reasonable to first look at supply, cost, tooling, and procedures as potential reasons. It's like people romanticizing about Leo's endeavors toward ideal tone in his choice of woods, while it's more likely that he chose ash bodies because they were the cheapest hardwood around and readily available in widths that would minimize need for joining. Alder was later switched to not for tonal reasons, but because that in spite of it's slightly higher cost and less availability in good widths, the cost and labor of increased joining was found to be outweighed by the savings in finishing, as it didn't need pore filling. Necks were 4/4 flat sawn maple - no need to guess on the motives for that.
Like Fender, Gretsch was a production shop, and the cost in time and material to intentionally seek out cuts of wood at such a seemingly senseless angle would seem unlikely. It may just be chance that seems close enough to consistent that we
want to believe there's some magical motive behind it. Or it could be a consistent pattern due to source of materials, tooling, or the some routine habits of a person who cut and separated brace stock all day long.
There are a lot of reasons it could be the way you've observed, but I would be very careful about wishful speculation (especially if you're writing a book). If you do find it so consistent as to be statistically unlikely to be random, I think you would really need come up with a few hypothesis to research, which I'm sure could be quite a challenge.