Warning : much of this is speculation, both on Zorian and the readers' side. Information accurate up to chapter 55 (Start of Arc 3).
We first learn of the legend of the Gate in chapter 33.
"The Sovereign Gate is an artifact supposedly dating back from [Shutur-Tarana Ihilkush]'s time. This story claims he either made or found a doorway into another world. Having found he did not age at all while on the other side, he spent 11 lifetimes there, learning their secrets and honing his skills. Eventually, he grew homesick and decided to go back home. Once he was back in his own world, however, he found the doors forever barred to him. He stored the Gate in his royal vault, there to wait for a worthy successor who would repeat his feat and usher the empire in a new age with the wisdom gained from the other side."
The Sovereign Gate is not mentioned again until chapter 54 and the end of arc 2, after Zach and Zorian learn that only one Branded One may leave the loop. There, the memory package of Spear of Resolve, the aranea matriarch, is finally opened to reveal memories which Zorian still cannot understand, as well as a message from Spear of Resolve informing them that she originally intended to be the one to exit. This having failed, she informs Zorian that the exit is located in a governmental facility deep in the Hole. She also informs him he may need to find a 'Key' to open it, consisting of the first Ikosian Emperor's five treasures : a ring, a crown, a staff, an orb and a dagger.
It turns out that the Gate is merely a means for the Branded Ones inside the loop to communicate with the incarnation of the spell, which calls itself the Guardian of the Threshold. The Key is not needed to summon the Guardian. The Guardian informs them that the Controller has already left and that the gate is barred. However, it also calls both Zach and Zorian 'Controller', which means that the Brand is in fact a means of controlling the Loop.
The Ghost Serpent believes that the Loop may only be initiated every four hundred years, during a planar alignment. It also believes that the Key is needed to do so, and believed that the Gate, the Key, or both had been lost.
Some light is also shed on how exactly the Loop works : a blueprint, a snapshot of the planet and possibly beyond, is taken. A copy is created within a pocket dimension, including everything from dirt to souls, but excluding the spiritual world (angels and demons). We can assume that the user's soul is given the Brand, else the Loop would simply run its course and end without any kind of progress, since no Controller would exit it before it crashed.
The Brand is a means of controlling the Gate, and warning it that the bearer of the Brand is not to be rebuilt during the restart. Zorian describes it as designed to be interacted with, and bearing multiple soul-triggered switches. We currently know the function of several of them: one of them can end a restart, regardless of whether or not the bearer is alive and/or not the primary user of the Gate.
Another switch only triggered upon physical contact with the Gate, and summoned a humanoid representation of it (the Guardian of the Threshold). The Guardian, based upon its confusion over the expression 'time loop', yet subsequent use of it, is capable of linguistic adaptation. The Guardian has a limited ability to talk.
He first explained that their souls were safe while talking to him, although if their bodies were killed the loop would end.
The guardian later explains that the loop typically ended upon the end of a month, or a multitude of other conditions, but only the maker and his agents are authorized to know what said conditions are. They explain that this is because due to minimal divergences this is not seen as murder. They explain that the looper has had their soul removed from their body and placed in the loop. They also explain one of their purposes is to minimize spill out of people in the loop to people outside. It explains that the soul marker is given by the Key, the Maker or its agents. It explains there is some way to do temporary six month markers that make a person retain their memories and soul across loops, and to permanently erase someone from the loop.
It later explains that there is enough power left for 52 more loops, 4 more years. The loop was triggered early, far away from the planar alignment, and so is more magically expensive. The loop has been active for 967 loops, 30 years of time, 11 days on average per loop.
A third marker control gives the Controller an indication of how many iterations remain until the end of the loop, and a fourth detects the presence of nearby pieces of the Key. This last ability has an unknown but quite limited range; however, its directions appear to be fairly specific, as Zorian was able to use it to track and locate Quatach-Ichl during battle.
Zorian is also aware of at least one marker function that is broken in his own marker, but working in Zach's. This feature causes the loop to automatically reset if it detects "significant tampering" with the Controller's mind or soul, including when the Controller dies. The broken trigger has mixed implications for Zorian: he is in more danger from necromancy, but is able to undertake actions that Zach cannot, such as advanced soul perception training that involves temporarily removing his soul from his body.
The Key Edit
The Sovereign Gate may be opened by using the Key, an object consisting of the first Ikosian Emperor's five treasures: a ring, a crown, a staff, an orb and a dagger.
The orb was lost in the jungles of Koth. Damien, Zach, and Zorian worked together to find it. They found it hidden in a cave underneath a Chameleon Drake nest and guarded by a magically enhanced hydra. They finally found it as an invisible space in the air. It is revealed that the orb can transform between a dimensional pocket and a small glass ball containing a palace. It seems that the orb was used as a sort of portable palace.
The dagger is held by the Ikosian Royal Treasury. Zach and Zorian attempted to steal it but were unable to penetrate the treasury's security measures.
The location of the two remaining artifacts is unknown exactly. However, Zach and Zorian's research indicates that one is lost in the Xlotic desert and the other was stolen and taken to Blantyrre.