Here's my theories on the gap. Golarion at some point in time would have developed the means to travel through space. But if I'm not mistaken this event would have likely taken place roughly 300-400 years after the war of immortals.
Here's why I think this. Reason one the rate of technological advancement. Especially humans would have been the pioneers of these achievements. They would be introducing the technology in the Inner Sea regions.
Triune hasn't come into being yet. Let's keep in mind that Golarion is behind when it comes to technology compared to Androfia and Earth. Being a high magic world would definitely influence Golarion's ability to produce technology. Like computers and smart phones lol.
Reason two space technology isn't even a thought because of the Elves who already established ancient gates to thousands of worlds. May I remind you that elves are most likely going to be the ones reluctant to adopt technology being that they have well established beliefs in preservation of nature. They're going to be a little bit more fearful at first. Humans who already adapted to life around magic would have to adjust to the changes.
As for the next 200 years or so humans in Urban environments would have already fully embrace technology. As for the dwarves. They're probably going to look at technology as a great asset because they love mining and anything that could improve their work culture is a huge plus for them.
Elves would later adapt seeing their younger kin start dabbling in tech and see it as a major benefit to better improve the ecosystems.
I think Absolom is probably going to be the first nation to embrace technology for the betterment of it's security and society.
Before I got off topic I think that The gap occured right around the year 5020 AR.sh