Selvor's gate wasn't actually a gate. It was a dam. And it wasn't inside Dylann, it was very much outside, pretty far downstream in Selvor. This made the relationship between Dylann and Selvor a little more interesting than with the others, since Dylann's Gatekeepers were actually operating on Selvor soil, and their dam was used for their own needs.

Thus the Gatekeepers acted only as sentries, controlling who came in and went out, with no control over the dam itself. Since this is where the river exited, there was a tremendous amount of water rushing around with a lot of power to be gained, and everything else that comes out of a rushing river. During flooding periods the Selvor gate was very busy with all sorts of rescue operations, and if relations between the two powers hadn't been friendly things could have been a mess. But no one doubts the integrity or neutrality of Twydion's people and Kaverna was forbidden from seeking power for itself, so things went along just fine.

Selvor's culture is very similar to that of ancient Egypt. Much of the settlement lay along the river, which flooded every spring leaving the ground rich for planting. The gate was named for the ruling family which hadn't changed in thousands of years. When the Betrayal happened Selvor suffered the worst of any gate. Because of the river there was no actual closing. The river flooded, and everything that was evil in Dylann poured into Selvor. The entire civilization was destroyed. Any survivors fled so far from the source of the darkness that flooded their land that they were never found during the restoration period, and Selvor remained a horrible wasteland.

Wow. So instead of this gate closing, the gate was just completely destroyed and the land laid waste?

Basically, yes.