Gore Nests are portals from Hell to Mars. They have to be cleared to remove lockdowns of certain areas in the game. They are often lightly guarded but taking one out calls in more demons from Hell. There are also in Doom Eternal, although they are for the most part in the Slayer Gates. There is also the Super Gore Nest, which is a massive one that can summon entire legions. It is also in the place where the initial invasion began.
Studies of demons upon entering this dimension have shown that their conduct is not purely vindictive - there is a method behind their actions. When a demon captures their prey, the fresh kill is taken to a temporary ceremonial site where arcane rituals are performed on the pile of blood and gore.
When enough corpses have been gathered, the ceremonial site becomes a "Gore Nest". These sites, imbued with Hell energy from the rituals, act as umbilical cords to Hell. Extreme caution must be taken when approaching a Gore Nest. Attacking the nest, or indeed any demons within close proximity to the nest, will act as an alarm and siphon more demons from Hell.
Gore nests appear to actually be a sessile animal organism of some kind like a coral reef, appearing to consist of a dinosaur-like head with a large open mouth lined with teeth in which lies a large spherical glowing red portal. Skulls and other body parts provide the underlying material for the "reef". On the right side of the gore nest "head" is a large yellow pustule which looks like it might be the creature's eye. This pustule encases the gore nest's heart, which keeps the organism alive, and which Doomguy rips out and crushes in his gauntlet when he destroys one. The Gore Nest also whispers loudly and ominously, and this noise is often the first sign the player has that there is one nearby.
The Mancubus has a similar organ to that of the Gore Nest within its chest, visible when the player performs a Glory Kill on it, which suggests that it is in fact a heart in the animal sense, even looking somewhat like a human heart with three large yellow pustules on it.
Dismantling a Gore nest involves ripping out its fleshy heart, which is concealed within a large eye-like pustule. This causes the gore nest to fall apart and explode into the piles of flesh and gore it was made of. Unfortunately, at the same time it also lets out an eerie, piercingly loud scream that serves as a distress signal, and a task force from Hell will warp into the area to deal with the threat.