So space is big right? If you are here with us at /tg/ you probably know that quote from The Hitchhikers Guide about how mind blastingly big it is. Its SO big that it makes the speed of light look slow in comparison. The Milky Way for reference is about 100,000 light years across. Its about 4.4 light years from here to Alpha Centauri. If you somehow managed to accelerate to ten times the speed of light it would still take you a handful of months to get there, and that's just to our our closest neighbor. At 10xC it would still take you 10,000 years to get from one side of the galaxy to the other.
So why the crash course in science I hear you ask? Well because you need to understand the distance involved and how long it takes to travel them to really get a good sense of just how utterly fucked the Necrontyr were in their war against the Old Ones. In the old fluff for them the distances across the galaxy weren't too much of a major issue since they had inertialess drives that could propel them to many many times the speed of light, but this created a bit of a plot hole since if they could move that fast why did they need to go to war and why did they get out maneuvered and beaten? In the more recent fluff however the Necrontyr never actually broke the light barrier (or if they did it wasn't by a huge margin) and instead got from system to system by freezing themselves in cryo pods (which incidentally also fixed the problem of the Necrontyr spending huge amounts of their short lives twiddling their thumbs in space, but in a way they don't actually enjoy). While they were at peace the travel time between star systems was an annoyance, but a manageable one, but during war time the delay was crippling. The Old Ones could travel halfway across the galaxy in a matter of weeks via the Webway which meant that they could strike with relative impunity at the Necrontyr supply lines (star bases, agri worlds et al) and it might take the Necrontyr literal decades before they would even hear about it, much less get a ship there to respond.
Someone at GW realized that even after getting their minds crammed into tin cans this would still have been a major problem, even with the super powered C'tan since its very hard to kill something you can't catch and Webway travel will always be significantly quicker then slower-than-light travel. Thus the Dolmen Gates were written into existence. Described as a series of "living stone portals" (apparently Necrodermis wasn't good enough...?) created by the C'tan known as The Burning One the Dolmen Gates allow access to the Webway, but only for limited periods of time and to limited points. The Webway is apparently at least partially alive and can detect when there is a breach. When it does it seals off access to most of its paths and attempts to squeeze the intruders out. Thus if you access the Webway from a Dolmen Gate you can only exit it from another Dolmen Gate, and you need to be quick about it lest you get crushed. So they basically act as wormholes, one way in one way out. Access to the Webway, even in this limited capacity for was more or less the final nail in the coffin for the Old Ones since they no longer had anywhere they could run or hide.
The Dolmen Gates make an appearance in Battlefleet Gothic: Armada II during the Necron campaign and serve as their only method of moving from one system to another, although you can move from any given gate to any other you control. On one hand it means you only have one way into a system, and if its heavy guarded you are right up shit creek, on the other hand it means you can keep all your fleets at central points (like systems where they can be repaired quickly) and dispatch then to where they are needed when they are needed. The game also brings a good hint to solve the previous contradicting lore on Necron FTL, necron ships actually have inertialess drives, they allow them to instantly teleport to another location, however, their range is limited and a ship requires a considerable time to reload its drive, so while the necrontyr could have probably made their torchships travel faster than light thanks to its drive these were still heavily limited in range when facing the far more efficient and galaxy-encroaching Webway and thus making its control a major war asset, and if this sounds familiar to you then good, you are paying attention.