This chapter is not necessary to read in order to enjoy the story, but still explains a lot of things regarding the Gammadimension that aren't outrightly stated during the story proper.
The Gammadimension is a hodgepodge of the three main dimensions from the Neptunia series (Super, Hyper & Ultra). The dimension is home to many alternate versions of notable characters from across the Neptunia series, with the most notable changes being their stances in the plot - protagonists and antagonists switch their roles. Not every character is reversed, and even the ones that are still resemble their original counterparts.
CPUs in the Gammadimension can be born in two ways. When Share Energy gathers together in a single location, it can either create Share Crystals which can be used by mortals to become a First-Generation CPU, like with the Ultradimension, or it can generate Pure CPUs. Pure CPUs are commonly born within already established nations, becoming the CPU Candidates for the nation's next generation, but rarely, they can be born outside any nation, in which they become First-Generation CPUs similar to turned mortals, although commonly more powerful. Most CPUs in Gammadimension's history were mortals who used Share Crystals, but almost all of them never maintained a nation past its first generation. When a CPU Candidate is born, they carry certain aspects of the previous CPU. This is called the Inheriting Effect, and is why Gammadimension's current nations have begun to fall, as without proper guidance from a Tome or the Goddesses' Aide, a single CPU can end their reign with a negative as their Candidate takes over, beginning a downward trend.
Tomes are celestial beings tasked with recording the history of Gamindustri, like with Superdimension Histoire, but are also meant to keep the CPUs from corrupting. Only one Tome can exist at a time, and when a Tome dies, a new one is created out of Share Energy when enough is flowing through all of Gamindustri. The Goddesses' Aide is similar to a Tome, only that she is a lot more direct and only one has ever existed; Arfoire. As Goddesses' Aide, Arfoire is tasked with ensuring all nations that ever exist within Gamindustri are properly maintained and that every CPU runs their nations with peace, order and prosperity. Arfoire also has her own Hard Drive Divinity state since she is effectively the True Goddess from the Superdimension, only that instead of ruling all of Gamindustri she only helps keep every nation afloat. Arfoire's HDD form is powered by the total Shares circulating around the continent. Nations that do not have proper influence from a CPU gather very little Share Energy, so it is vital for CPUs to have enough of a hands-on approach with running their nations.
Gammadimension's Gamindustri's layout mimics the Superdimension's (Planeptune to the south, Leanbox to the east), but the geography takes inspiration from Hyperdimension's continent, although far larger. In the centre of Gamindustri lies Celestia; the divine land. Legend tells that Celestia was the birthing point of all of Gamindustri, and where the very first Pure CPUs were born. The entirety of Celestia is brimming with Share Energy, and the flora and fauna native to the land evolved to make use of it.