"Mercenary Ninja" is not in the list of possible values (ANBU, Cooking-nin, Daimyō, Hunter-nin, Jinchūriki, Pseudo-Jinchūriki, Medical-nin, Missing-nin, Ninja monk, S-rank, Sage, Samurai, Sannin, Sensor Type, Summon, Tailed Beast) for this property.
He and his brother fled the village with Zabuza Momochi when they failed in their attempt to kill the Mizukage, and thereafter began working for him as assassins.
In sharp contrast to his brother, Gōzu is described as being rational, level-headed and calm.
Gōzu had shoulder-length, wild dark-brown hair and dark eyes. He wears a rebreather that covers the lower half of his face and a large, clawed, poisonousgauntlet on his right arm which had a chain coming out of it that had the other end attached to his brother's own gauntlet. He wore a camouflage suit with bandages around his waist, dark-coloured, knee-length sandals and a ragged black cape. His Kirigakure forehead protector had a single horn on it.
Gōzu and Meizu are a paragon of coordination attacking. Gōzu's large metal gauntlets house a retractable shuriken chain running between the two of them, which they can use to bind an enemy and then slice them to pieces. In the event that the shuriken chain is neutralised, Gōzu also has a release mechanism to free him from Meizu for individual attacks. He uses the sharp, poisoned claws on his gauntlet to shred his foes. Gōzu was also skilled in stealth, being able to hide right outside the gates of Konoha without being detected.
At the start of the series Zabuza sent them to kill Tazuna. Using the Water Release: Hiding in Water Technique to surprise Team Kakashi. With Gōzu immediately launching his brother in front of him the pair are seemingly able to kill Kakashi with ease. As they moved on to attack Naruto Uzumaki, they were stopped by Sasuke Uchiha who broke the chain connecting the two brothers. Still intent on killing them, the brothers separated and attacked the team. They were however stopped by Kakashi Hatake, and it was revealed that he had used the Body Replacement Technique to escape their first attack. Both he and his brother were later tied to a tree so that they can be taken into custody.
Gōzu's name is written with the kanji for "business/deed/performance" (業) and "head" (頭), indicating a mindset focused on the fruitful outcome of his actions, in contrast to his brother's more emotional/irrational way of thinking.
According to the databook(s):
Gōzu has completed 313 official missions in total: 71 D-rank, 196 C-rank, 45 B-rank, 1 A-rank, 0 S-rank.