Now, think back to the interviews made by Gen Urobuchi.
In those interviews, he does showcase a clear knowledge and understanding on Kamen Rider, more so than many other writers in fact. He understands the horror, monstrous side of Kamen Rider. From the setup of the plot, he also understands Kamen Rider and its nature vs. society as a whole, and the implications of how horrible the world has to be for Kamen Rider to exist in the first place.
He has also once said that, he wants to recreate what he felt from watching and reading Kamen Rider Black.
If we reconsider everything in Gaim through that lens, what Gaim really is about becomes clear:
In the finale of Gaim, Kouta has received the Golden Fruit. He has become the Overlord of Helheim, the Man of the Beginning, and cast Helheim and himself from Earth to somewhere grow else entirely.
Earth can continue its existence.
But so does Helheim.
Kouta, as a Kamen Rider, has failed.
First, Kouta has submitted himself to Helheim’s rules and become a ruler, an authority.
The position of Kamen Rider as the king of the domain that birthed him is absurd, when you realize how the first and in fact most of every Kamen Rider is born from misery and as a result of despotism.
Yet Helheim claims to be representative of nature, of evolution. A stance that Kamen Rider has always sided with.
That’s because Helheim is simply not nature a all.