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.
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. 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 or brain, which keeps the organism alive, and which Doomguy rips out and crushes in his gauntlet when he destroys one.
The Mancubus has a similar heart-like organ to that of the gore nest within its own eye, which suggests that it is in fact the brain and not a heart, even though it looks somewhat like a human heart with three large yellow pustules on it.