Simon,
I submit the small diagonal is a design artifice, mostly. It really isn't necessary at all, as this photo of the same gate by the manufacturer without the small diagonal shows. The diagonal is also, you will note, thinner material.
That little diagonal is apparent mainly in British gates I see, and seems to be a design element no matter which Way the main diagonal goes. I rarely see it in the States. Some sort of Tudor echo, I surmise.
Here is what I consider an ill thought out (British) gate. The small diagonal seems merely added when the builder found the horizontals sagging. The verticals were added when the top horizontal bent. You can justify almost anything in modeling.
But maybe you just aren't used to seeing the diagonal down, so I'll condition you to the idea:
A common retrofit here is an adjustable metal frame that is small diameter material, and then the only way to properly use it is with the diagonal extending downwards from the top hinge:
It's not very elegant, but it is bombproof if a soft wood that doesnt hold fasteners well like cedar, redwood, or treated hemlock or fir is used. ( that gate appears to be Western red cedar with treated ponderosa pine horizontals - both very soft woods).
Or if the whole gate is small size material, it likewise is always in that orientation.
The extreme example is if the diagonal is wire or metal strap, of course.
This isn't to say that the diagonal isn't built as you submit, but I think that is mostly a design choice. The "A" look of a double gate is pleasing, I will admit.
When considering the stresses a gate is under, wood is quite strong enough in tension for the task, but perhaps I am being defensive, because given the choice, I preferred the "diagonal down" method when I built gates in my 40 years as a carpenter. The bolts of the top hinge can often also go through the diagonal for added strength (as Richard has modeled). So made, the stresses tend to close well made joints instead of pushing them apart. Then they don't sag over time, especially if softwoods are used, and need fewer fasteners at each intersection for long gates than the other method. Richard's
Gate is rather well engineered, with one minor exception.
Note the long top hinge strap on several of the gates above that also keep the top joint together. That is the joint with the most stress in a gate. If he were to make a change, that is all I would suggest. But there are as many examples above without the long strap?
With the diagonal up, the diagonal, being the longest element in the gate and often thinner material, will bow in compression when the gates are very wide without secure intermediate fasteners and the gate will sag anyway.
But if you buy a gate pre-made at a big box store here, the diagonal will come one way, and whether it goes up or down will depend on where you decide the hinges need to go.
