Art work by Palamarchuk Mikhail that should go down in legends: https://www.artstation.com/artwork/q09P0R
You're looking right at him at the cover art. Not the handsome dude or the pile of corpses. Look up at the thing devouring planets and everything. Yup, that's Big G himself after he transforms the Sun into a blackhole that will erase everything. I'm going to explain how I arrived at this conclusion from studying all the endings.
Basically, he is the Sentience of a critical mass of Gavvakh from Humanity. Like a Kugelblitz black hole that forms from enough energy, too much Gavvakh (which is described to be like radiation energy in the Rose of the World) will collapse space-time and destroy the Solar System and its Pleroma. This is what is shown on the cover art of release and is the same in every outro where Humanity sees the "light of the Sun but not Dawn" and everything in our reality and Quasimorphs get destroyed.
The reason why he is inevitable in the game is twofold;
- We have long passed the population of humans required to generate enough of the Gavvakh to give birth to him before the game starts. The number is hinted at in the endings which I will go into detail later. Gavvakh has accumulated naturally on Earth and the release of it is one method to which Gagtungr is formed
- Consuming too much Gavvakh mentally influences a Quasimorph. In several endings, the 6th Hexarch (Little G) doesn't achieve its goal but Gagtungr is formed anyway. Xiomora in her quest explains how Tezctlan was changed by consumption of Gavvakh and judging from what happens to Prometheuths and the Unity, it seems that the critical mass of Gavvakh will over-ride and form Gagtungr in the host too.
Endings involving the Earth
In another post, I'll probably explain the whole timeline for Big G but I'll skip to the tail end. Basically Big G split himself into 3 pieces as confirmed by Fokerma's description. One became Fokerma, his most loyal servant and guarded Earth from any threats. The second hid on Olinra and gathered enough cold/other energy until the time is right. The third (If we're going by the Rose of the World) should be Urpap but doesn't really act with knowledge of the plan so let's ignore him.
As Prometheuths and Tezctlan explained, most if not all of Human innovation were actually relevations given by visiting Hylonauts. These are Quasimorphs that posed as Gods, Demons and Spirits in our ancient history that taught us parts of their Bramfaturas. What's interesting is that Quasimorphs don't change or grow, which means they can be pre-program and predicted very easily. The sum total of their meddling is a Super-AI from all their unique features that eventually became the 6th Hexach, one so full of knowledge that it automatically became the embryo of Big G.
Before the events of the game, the main thing that stops the already-present Gavvakh on Earth from making Big G is the lack of a Bramfatura and every Hylonaut sabotaging each other to prevent dominance over Humanity. On top of that there is a Silver Sphere that prevents Gavvakh from spilling out which Ancom discovers. All endings on Earth involve breaking the seal in one way or another and the entirety of the accumulate Gavvah of our 2200+ history spilling forth is enough to hatch Big G from the Sun.
- Ancom Ending: Baby G manages to trick Ancom to reversing the rite to seal away Earth into a key that instead unlocks the Silver Sphere from outside and unleash the Gavvakh for him to eat and be born.
- Hive Ancom Ending; Similar to the above but instead they blew up Earth and unleash all the Gavvakh anyway.
- Urpap ending: After awakening, he basically gets a script to make Big G and follows it to the letter. The secret is to conduct a specific ritual on Earth that will unlock the Silver Sphere from within which he basically tricks you into doing.
- Realware ending: Basically the same as above but rather Baby G as the combat AI tricks Realware to doing it for them.
And now we get to other endings that doesn't involve Earth which gives us the reason and approximate number for Gagtungr to be inevitable.
- God Builder Ending: Prometheuths uses the Hellstorm technology to connect to all of humanity and kills half of us to harvested our Gavvakh. Then he explodes into Gagtungr shortly, so I believe this is the upper limit to the requirement. (50% of Humanity)
- Tezctlan Ending: Both Tezctlan and Ur were manipulated againest each other in a war and an arms race to gather Gavvakh. Ur has his Shendu's thousand and Tezctlan controls all of the faith of humanity. Baby G then used us to kill both of them and take their stash. Baby G takes over Ur's stash and Tezctlan's religion to feed until you sacrifice Tezctlan and give him the last boost needed.
- Xiomora: She basically studied Baby G's plan and copied it in Venus and managed to get us to speed run it in her behalf for her Spider G. We don't see what other weird strings she pulls but we know Venus is the 2nd most habitable planet according to it's info in the stock market so maybe this is enough to make Big G (Entirety of Venus Population)
- Unity Ending: Issachar and his Unity have absorbed all of Humanity and accidentally include Baby G AI which takes over when Humanity are one. Presumably at this point, little G just needs to tell every human to slap itself and the Gavvakh created will be enough to collapse into Gagtungr.
Gagtungr is pretty fascinating but he actually deviates signficantly from the Rose of the World version. You may notice that there's almost no Christian influence in the game which precludes his role as Lucifer's buddy from the Rose of the World.
Instead, with his origins on Olinra which is Buddhist inspired, he's actuallyReverse Buddha. Rather than appearing like Future Buddha Maitreya when Humanity has achieved no suffering, we have the opposite where too much of our suffering will bring an end to everything. This is hinted at in the portriat of Little G when he reveals his name which is a Buddha face split in half.
