The Gillman is one of the alien footsoldiers that served in the Second Alien War, generally used as frontline troops early in the game. Though they are part of the alien army, upon studying Gillmen corpses and interrogating captured units, it is clear that they are not true aliens, but indeed native to Earth. Or at least they were...
The Gillmen populated the seas of prehistoric Earth, when reptiles dominated the young planet. They are an ancient offshoot of humanity, a species of amphibious reptile sapients. They are a full-fledged race with both male and female genders(though we never see any Gillwomen). They virtually disappeared at the end of the Cretaceous, when the Alien Colony Ship T'leth crash landed and wiped out the last of the dinosaurs and many other prehistoric life forms. The Gillmen were one of many prehistoric creatures that were harvested by the aliens for their troops. When the aliens started the Second Alien War, the Gillmen were under alien control, forced to battle against their Human brethren.
Gillmen are about the same size as a normal human adult, with slit-like eyes, green scaled skin and lizard-like faces. They have more physical prowess than the Aquatoids, but not enough to compete with the Tasoths or even the Lobstermen. They can take slightly more damage than the Aquatoids, that is if you are still using Jet Harpoons. If you've got anything better, they won't pose any challenge. And although they have slightly better stats then the Aquatoids, high-rank Gillmen lack Molecular Control. For some strange reason, Gillmen never carry Sonic Cannons.
They are accompanied by Deep Ones and Xarquids on Terror Missions.