Gyro is a cocky and flamboyant executioner marked by the harshness of life, who participates in the Steel Ball Run for the sake of a young boy.
Gyro initially appears as a cocky and aggressive contestant in the Steel Ball Run. Unlike other competitors, Gyro looks to seize the first place aggressively, rushing ahead of everyone then fending off anyone who challenges him. Gyro is a rude man, deriding the vast majority of those he meets with insults and mocking commentaries. Even with his closest friend Johnny Joestar, Gyro doesn't mince his words and sometimes argue with him. The only person for which Gyro had any degree of respect is his stern father Gregorio Zeppeli. Gyro's cockyness and rudeness extend to his fights, during which Gyro is prone to mock his enemies in some way or say something cocky. However Gyro doesn't have the same ruthlessness as Johnny and initially doesn't seek to kill his enemies. Gyro is particularly proud of his mastery of the Spin, a family technique involving the rotation and throwing of steel balls to various effects and hails the Spin as the acme of human skill, this pride is latter justified when the Spin proves to be able to defeat even the Stand D4C - Love Train.
Gyro nonetheless possesses an altruistic side, and likes to help anyone who needs his assistance, most notably the paraplegic Johnny to whom he taught the Spin technique and helped acquire the Holy Corpse Parts, or Marco, the helpless child whose death sentence Gyro seeks to appeal by winning the Steel Ball Run. Gyro would often clash this side with the authority of Gregorio Zeppeli, who disapproved any display of sentimentality in life and ultimately defied him by going to America.
Early in the Steel Ball Run, Johnny has said that Gyro didn't have the drive and determination necessary to get things done. The jockey argued that because Gyro inherited everything he had as an upperclassman and heir of the family Spin technique, he didn't have the kind of selfish hunger born from suffering which was crucial in the race. True to that, Gyro Zeppeli initially raced out of altruistic concern for Marco, and primarily relied on tricks to prevail in the race. During his battle with Ringo Roadagain he bent his original motivations into a more personal goal, the path to see for himself whether Marco deserved to be executed and and who exactly the Corpse was. After that, Gyro switched to a slightly less cocky behavior, and focused seriously on his battles. In his horse riding, Gyro switched to a more personal style, instinctively choosing his own route instead of the "shortest path", enabling him to even outmatch Diego in the race.
Out of combat, Gyro also possesses an eccentric personality, and is particularly flamboyant. Gyro has a penchant for gags involving puns or an inexplicable brand of humor, frequently telling such jokes to Johnny in the spur of the moment and created a song about cheese, singing it to Johnny and being happy that he appreciated it. He has a fondness for dramatic gestures and quotes, saying that the Goddess of Victory was riding with him and that she would retract her favor if he ever let another woman ride with him. Gyro is a womanizer, one flashback narrating an affair with a patient, and has tried to flirt with other women. However, mimicking his own father, Gyro can be stern and dignified when he talks about the Zeppeli Spin technique. The first time Gyro and Johnny meet Lucy, he refuses to let her ride with him and states that "no woman (will) ride my horse". He later explains that "Lady Luck" would be jealous if he let "another woman" ride with him.
Heavily influenced by Gregorio's teachings about the uncertainties in life - using a " tennis ball over the net" metaphor - and the Steel Ball's power being nothing short of miraculous, Gyro has a somewhat optimistic outlook on life and is convinced that he can overcome setbacks, even if they'd need a miracle to overcome, as long as he follows a virtuous path. Nonetheless, following the "tennis ball over the net" belief, Gyro is aware that failure is just as likely. This ideology is repeatedly somehow confirmed with numerous coincidences and miracles helping him throughout his journey and alternatively working against him, culminating in Marco being freed without his intervention but dying of a cold later.
- Gregorio Zeppeli: Gregorio Zeppeli was Gyro's father, and was the only person to ever command respect and obedience to him. Their relationship was probably cold due to Gregorio wishing to throw away sentimentality from his life, thus never had a compliment for his son, nor did he give him gifts and send him letters. However Gregorio was still Gyro's Spin teacher and direct superior as surgeon and executioner, thus has authority over him, although Gyro still disobeyed him behind his back.
- Johnny Joestar: Gyro and Johnny are friends and partners in the Steel Ball Run and also master and apprentice in the art of the Spin. At first Johnny's determination got Gyro's interest, causing him to help him figuring out how to mount his Slow Dancer and start the race. After the first stage, Johnny wins the respect of Gyro, thus Gyro decides to teach him the Spin and they become partners for the rest of the race. As the stages progress their friendship grows. Despite their opposing personalities, causing regular clashes as to how to take on a difficult situation, Gyro and Johnny ultimately get along well and share many moments of friendship, culminating in them telling each personal secrets. Gyro notably likes to jest with Johnny and tells him bad jokes or miscellaneous songs that Johnny frequently pretends to appreciate. Due to their regular battles, Gyro and Johnny fight well together, and see each other as valuable allies. Their friendship is such that any one is ready to put himself in danger for the other, and Johnny notably abandoned his Corpse Parts to save Gyro's life, and has cried multiple times for him. When Gyro died, Johnny was completely devastated and Valentine exploited that bond to promise Johnny that another Gyro would walk this universe. Gyro's ghost salutes Johnny one last time and the latter would bring back his corpse to the Zeppeli family. Gyro's continuous support helped Johnny get out of his depression and enabled him to begin anew.
- Lucy Steel: Gyro and Lucy became allies, but Gyro had a cold attitude toward her, refusing her request of helping her and her husband and instead suggesting that she risked her life trying to steal the Heart from Valentine. Gyro nonetheless acknowledged her bravery and resourcefulness and entrusted the Right Eye with her, later asking Wekapipo to save her from Valentine.
- Wekapipo: Despite meeting in the Steel Ball Run as long-standing enemies, Gyro and Wekapipo have a certain respect of each other's ability as both are proficient Spin users. Gyro revealed to Wekapipo that his sister was alive and well, having saved her life once. This revelation earned Gyro Wekapipo's help during the search for the Corpse Parts.
- Mountain Tim: Gyro only saw Tim as a rival in the Steel Ball Run race, deriding his hat when he first saw Tim. However, both come to see as allies during the fights against the Boom Boom Family and Oyecomova.
- Diego Brando: Gyro and Diego are bitter rivals, as both repeatedely tried to outdo each others in the Steel Ball Run. Gyro thinks of Diego as the greatest threat for his goal of finishing first in the race and never ceases to antagonize or insult him. Diego won't hesitate to kill Gyro, but Gyro's moral code stops him from doing the same, and he only wishes to beat him up.
- Hot Pants: As one of the competitor in the Steel Ball Run and a rival in the search for the Saint Corpse Parts, Gyro views Hot Pants poorly. Their first meeting consisted of Hot Pants attacking Gyro for wrong reasons, and Gyro never let up his antagonistic behavior after that, regularly insulting the horse rider and once stealing her food. Later on, Gyro remains in the dark about Hot Pants' true identity, while Johnny was able to forge a more understanding and sympathetic relationship.
- Sandman: Gyro and Sandman are rivals in the Steel Ball Run race, but initially act friendly with each other. Sandman bore no ill will against Gyro and repaid being made winner of the first stage in Gyro's stead with critical information during the fourth stage. However, the next time they meet, Sandman now worked with Valentine and tried to force Gyro and Johnny to give their Corpse Parts, going as far as trying to kill them. Nonetheless, Sandman was merely indifferent to what he saw as the struggle between white men.
- Pocoloco: Gyro thinks of Pocoloco as a dangerous rival and mistakes the latter's luck as talent, while Pocoloco sees Gyro as a mere stepping stone in the race.
- Boom Boom Family: Gyro was initially cold and cautious to Andre Boom Boom, and he was later proven right to act in such a way as Andre, along with his brother and father, attacked. Gyro thinks the whole family as idiots, though holds regard for their dangerous Stand abilities.
- Oyecomova: Oyecomova was an Italian assassin who attempted kill Gyro, Tim and Johnny, only to be defeated by their respective Stand powers.
- Ringo Roadagain:
- Funny Valentine: Valentine was Gyro's enemy in the search to obtain the Corpse Parts. Valentine eventually killed Gyro.