Gapspace
On the Nature of Gapspace

A possible view of Gapspace as seen from the surface of Grand Convergence. Do not touch.
Despite the menace of entities spawning from the interaction of Gapspace with the reality of Grand Convergence, there is no eldritch horror behind the curtain. There simply isn't any room. Gapspace contains nothing. No concepts of space or time function within it's boundaries.
Gapspace is extremely difficult to study because it actively rejects attempts to provide data. Readings rapidly begin to dissolve into unintelligible strands of nonsense, while attempts to measure physics or energy phenomena collapse on themselves. It is whispered that the very act of research is tainted, and those who delve too deeply into the study of Gapspace are consumed by it.
Of what little is known is that Gapspace is perceived as the color of 'Cosmic Latte' -- An off-white marked by a faint greenish tinge. Some races perceive this differently and there are many varied ideas about how sentient life interprets the visual data ascribed to 'Nothingness'. Within the relative safety of a Spireway, one is often shown sparkling, chaotic vistas during the transit which blur and blend like a fractured kaleidoscope as they move.
Hazards of Gapspace
Gapspace, as a container of no physical rules or temporal reference, remains extremely dangerous. More than one brave (or arrogant) adventurer has met their demise thinking they could simply fly through it, only to vanish without a trace. Perhaps they are caught somewhere like an insect in amber. Perhaps.. that is the best outcome.
Yet exposure to Gapspace is not instantly lethal. A few minutes is not crippling, but the distinct feeling of your personal physics and frame of reference dissolving is a sensation most would pass on the opportunity to encounter. There are even ways to reinforce an envelope of physics around an object or person, making short term or emergency exposure possible yet extremely risky. Only in the most dire circumstances should one leave the safety of a Spireway, and no documented or observed attempts to cross from Grand Convergence to another World, no matter how 'close', has ever succeeded.
Who knows what happened to these doomed expeditions. After all.. Gapspace is not the only danger.
Waystations

One of many possible Waystations along a Spireway. Don't stray too far from the road.
These are stable locations created as a natural side effect of the Spireways stretching through Gapspace, especially if they branch to connect through multiple worlds along the same line. These locations are often where enterprising inhabitants will set up what amounts to a 'Tourism bureau' or a 'Tourist trap' with all the same services of a rest stop, restaurant, general store and hotel. They can often be used as a somewhat secure home base for those intending to explore uninhabited worlds or a Beacon that has landed in a dangerous location.
Manipulating Gapspace
Despite the efforts of a thousand years or more, none have managed to replicate the functions of a Spire well enough to create their own Spireway. However, there has been much research into various ways to reinforce and improve the general stability of an already established Spire. Some have created methods to augment specific connections to Beacons as a way to assist with trade (Beacon Keys), or the creation of spells and objects to locate Beacons or attempt to reinforce one's personal reality so that it does not unravel with contact with Gapspace. (This.. sadly, ranges wildly in effectiveness...)