I would only add that to the untrained eye (by which I simply mean one that isn't used to looking critically at tartan design) a lot of tartans look very similar simply because they use the same colors. This may lead one to beleive there is a connection when in fact there isn't any.

For example, a lot of people think that the MacLean of Duart tartan looks just like the Royal Stewart. They are both red-based tartans, and since most people are familiar with the red Royal Stewart, they tend to associate any red-based tartan with that one.

In the case of Gunn and Barclay hunting, the basic colors are blue and green with a little red. But this is also true of hundreds of other tartans. Which is why you yhought Davidson so similar.

If you look past the colors, though, to the design itself, you'll see that Davidson really has little in common with these other two. Barclay and Gunn do have certain similarities. Both have equal amounts of green and blue, with the green bisected by a red line, and the blue bisected by a green line. But the Gunn also separates the green and blue portion with a wide section of black.

Gunn actually has more in common with Mackay, and this makes sense as the clans Gunn and Mackay were neighbors. In this case, I think the similarities between Gunn and Barclay hunting are a coincidence. I think in the case of Barclay, the hunting is a simple color change of the more popular Barclay Dress tartan, which is the same design, only in yellow and black. Both come from the Vestiarium Scoticum, which was published in 1842. Most of the tartan designs in that book are very simple grid-like patterns, the result of them being designed by artists drawing on a page rather than weaving in cloth.

Aye,
Matt