[SS Tim]

There are several issues here with the material direction.

First is the material direction difference cited by oldchevytrucks.com which most of us have seen and relied on as correct, myself included. But that was also two years ago at the beginning of my Highlander Plaid research. Deeper digging has revealed that information page to be incorrect and incomplete.

Second is the fact that the material is very indistinct and in the right lighting conditions the stripes can appear to go in either direction. This is most noticable in the grey due to the lessened contrast of the black, white and the grey halftone. This effect can be more pronounced on the Blazer/Jimmy bucket seats simply due to less stripe area to set a visual direction.

Third is there are few original GMC trucks to compare to. Mine (a glovebox door/SPID and pictures) which I was fortunate to have been given is of little value. The seat cover had been replaced along with the seatbelts. In addition the interior was completely redone poorly and apparently at random. However it did document that the trim codes were the same as Chevy Highlanders and it has the original dark fawn interior color under and surrounding the SPID.

Luckily there are other vehicles.

Most people go to the only reference they have seen of the GMC plaid, the brochure picture. This is the same picture I believe was the basis of oldchevytrucks notation of a rotated material.
The problem is there exist Chevrolet brochures that show the same apparent horizontal primary stripe.
As an example here are the two side by each. This shows they may have at one time been intended to be horizontal and changed for production or more likely a simple case of the plaids changing with light angle and contrast.

Since the trim codes are the same and it would be unlikely they would use the same code for different products going down the same production lines, I feel they were all in the confirmed normative (to Chevy Highlanders) vertical primary stripes.

But there is more. Like the orange GMC above with vertical stripes there is also a Jimmy project that we have access to showing the original front and rear covers (with vertical stripes) and a new replacement cover that the trimmer had made with horizontal stripes showing a pronounced conflict. Interestingly the stripes were run that way as a convenience to the trimmer not in an attempt to match a "known correct" (horizontal stripe) or even to match the original worn cover.
So here are a series of pictures to get you started...