Gateways What Lay Beyond is a compilation of the conclusions to all of the Star Trek Gateways series. Personally I only read "Horn and Ivory" which is the conclusion to the Deep Space Nine Gateways series.
There is nothing astounding to the conclusion. First of all, I will warn everyone that spoilers follow. So stop reading now if you want don't want to know what happens.
The whole story is focused on Kira, none of the other DS9 crew plays a role. In the last Gateways book, we find Kira stranded on a deserted planet in the Delta Quadrant dieing of theta radiation poisoning and Taranatar recovering from a brutal fight with a Hirogen. We also know that at the end of the last Gateways book, Kira staggers into a gateway that keeps flashing between two places. We would expect something to happen like her being returned to DS9 or Bajor but that's not quite what happens. After staggering into the Gateway, Kira is transported to a familiar planet 30,000 years in the past. The rest of the story has nothing to do with solving the mystery of the Gateways, it only opens up more questions. Not until the final chapter do we find out what happened with the evacuation.
It's a nice litte story, but you can easily skip it and go on to the Deep Space Nine Gamma series.