Gigantomachia was a member of the League of Villains before even Tomura Shigaraki was. He once served as All For One's bodyguard, during his government in the shadows, although it is ignored for how long.
Foreseeing his inevitable downfall, All For One decided to hide Gigantomachia in the mountains, with implications that All For One groomed it into a safeguard in the case that the situation of the League comes to a point that they are at risk, and warns Kurogiri to use him once he believes to no longer be capable of ensuring Tomura's safety. 
Months before the start of the tests to access the U.A. High School, Gigantomachia was walking through the city towards the Springer's Hero Office, but he got lost. He threateningly asked two middle-school girls for its location, but the girls were too terrified to utter anything. Gigantomachia took their silence as offensive and put his hand on a building, which caused the building to start breaking down.
After All For One’s defeat, Kurogiri informs Tomura about the "power" All For One left behind and wondering if he should retrieve it to advance the League of Villains.Kurogiri began searching for Gigantomachia in the mountains. However, Gran Torino and Naomasa Tsukauchi have managed to track down and find Kurogiri with the help of eyewitness reports and have subdued him. Kurogiri asks Gran Torino if he knows of the rumors about a "ruffian" appearing on the mountain where they are at and the reason he is here is to speak to the "ruffian". Gran Torino would prefer Kurogiri to tell him all about it behind bars. Suddenly, a huge noise reverberates in the area which catches Gran Torino's attention, and Gigantomachia makes an appearance.
Gran Torino and Naomasa are seriously injured and the task force suffer severe casualties at the hands of Gigantomachia, but they manage to take Kurogiri into police custody, at the cost of allowing Gigantomachia to escape, after having completely destroyed a large portion of the mountain. Later, Gran Torino phones All Might to inform him that they captured Kurogiri, and also to warn him about the threat posed by Gigantomachia, which he defines as a walking disaster.
Some time between late September and early October, before the Hero Billboard Chart JP, Gigantomachia finally meets Tomura and the new League of Villains. Tomura expresses glee as he realizes that Gigantomachia must be the power Kurogiri mentioned to him. Dabi and Mr. Compress are stunned by his appearance.
After setting down his radio and confirming Tomura's identity, Gigantomachia challenges the League to see whether they are worthy of his loyalty. Gigantomachia easily prevails over the League managing to incapacitate Himiko Toga, Twice, and Mr. Compress.
He is dismayed at their performance, tearfully crying out that Tomura is too weak to be a successor, calling the League unacceptable, while questioning All For One's motives. He comments that wants to accept Tomura but can't. A tearful Gigantomachia proceeds to smash the ground, causing all League members to lose their footing. Gigantomachia's sheer power ends up destroying the majority of the landscape.
During the confrontation, Daruma Ujiko speaks to Tomura regarding his uncertainty in the League's current state and need for help. Daruma explains that Gigantomachia has unlimited stamina and did not require any modifications to serve All For One as bodyguard and confidant. He reveals that All For One hid Gigantomachia to make sure his legacy would live on through Tomura's command. However, the doctor agrees that Gigantomachia is loyal to a fault.
Before Gigantomachia continues his attacks, a voice recording of All For One plays from the radio. It effectively calms Gigantomachia as as he nuzzles and purrs at the radio. An irritated Tomura states that he does not want Gigantomachia. The Doctor responds with Gigantomachia can be useful and warps the League to a secret laboratory filled with multiple Nomus to continue the conversation.
After their conversation Daruma instructs Tomura to get Gigantomachia to submit. Daruma has the League, minus Dabi, warped back. A confident Tomura tells Gigantomachia that the 'future king has returned'. An amused Gigantomachia replies that a king should 'inspire dread, be admired and be strong'.
The fight continued and went on for days while he fought the other members, he ignored them and mostly focused on fighting Tomura who still possessed a stronger resolve. Gigantomachia called his efforts weak but was impressed by his determination.