Militarily speaking, these would be every bit as significant to tactics and strategy as the introduction of gunpowder or Traveling.

And it seems to be an involved process, and probably not easy to duplicate.

There is also simply using gateways themselves to communicate. Even (or especially) people who are too weak to make a gateway big enough to walk through can utilize it. I cannot imagine that sound is impaired by the size of the gateway it is passing through. Simply set up a communications center, and divide it into sections, each warded against eavesdropping and outside noises, and in each section, open & tie off a tiny gateway to a similar room elsewhere. This way, major locations and centers of power, or even military HQs, can keep in contact with each other. Simply set up such a room in the White Tower, the Stone of Tear, the Royal Palace in Caemlyn, the Sun Palace, the Black Tower, etc. You can simply have a person manning each mini-gate and writing down whatever comes through, or speaking out the messages he is given to relay. Nobles and merchants might even have similar set-ups, as long as someone breaks through the White Tower's abhorrence of doing useful work. If that is too awkward or risky or man-power-intensive a situation, replace it with gate-mail. Set up post offices but with mini-gates large enough to take a letter, that lead to a central hub, where it would be sent through a similar gateway to the appropriate office. Maybe there could be a central hub in each country, with the larger countries broken down into regions, and each regional hub having gateways to a post office in each major town. Larger towns could be broken up with offices in each neighborhood large enough to qualify, or wealthy enough to afford it. Elayne wants to start a "royal post" but why waste time with horsemen and wagons carrying sacks of paper on months-or-weeks-long trips where every mile increases the odds of it getting lost or stolen or destroyed en route? The worst that can happen with the gateway system is getting a letter sliced on the edge, which could be minimized by fitting some sort of wooden or metal frame inside it.
Basically, such a system would run like a post office does now, but without all the vehicles - the steps between a clerk sorting outgoing mail and another one sorting incoming mail would all be eliminated!

You DO know that Sammael was lying, right? He told Graendal that Rand agreed to their truce, when in fact he did no such thing. That means one of two possibilities - Sammael's method is flawed, leaving him to think he had a truce, which must have come as a huge shock when Rand invaded his country, and demonstrating a huge flaw in this method of communication, or else he was simply lying about the system, and it was NOT set up to relay an answer to him - that his presentation was all for show so that Graendal would learn of his offer and believe he made a truce with Rand, and thus submit to Sammael out of fear. So, I don't think this can be assumed to be a genuine technique for communication.