The Gonome is a "mature" form of zombie that appears in the Half-Life add-on game, Half-Life: Opposing Force.
While the precise reasons for the transformation are unknown, it appears that if a headcrab and its host survive for an unspecified period of time or are subjected to certain conditions, the zombie will develop into a Gonome. The host experiences a growth in overall body mass, and the gash in the chest cavity develops into a functional, vertical maw. The Gonome also obtains the ability to spew organic projectiles from this maw.
While the host body appears to grow dramatically, the headcrab controlling the host appears to wither or deflate; the outlines of the host skull's eye sockets and nose are visible through the headcrab's skin.
Gonomes generally stay with groups of lesser zombies and headcrabs. If they observe any threat, they will first use their ranged attack. Should the enemy get closer, they will resort to manually attacking with their claws. They can sprint if they spot a distant target. In many ways, the Gonome's behaviour is similar to that of a Bullsquid.