Limiter

...No matter how much effort one puts in, every living being has an intrinsic limit to its growth. Too much power becomes unbearable and overwhelms its host, turning it into a mindless rampaging monster. To ensure that we do not enter the realm where we lose all purpose and ability to reason, God has set limits to the growth of every being. The mechanism with which growth is controlled is called the limiter.A limiter (リミッター, Rimittā) is a term coined by Dr. Genus for his theory on explaining Saitama's strength, which defies common sense. No matter how much effort is put in, there will eventually be a point where a being stops growing. According to the theory, this growth limitation is placed on every being by God so they do not acquire so much power that they cannot control or handle it; furthermore, it is possible for this limit to be removed somehow and an individual can achieve heights far greater than what should be possible.

Gyoro Gyoro is very aware of the concept of the limiter, although she does not use or even aware of the term as Genus was the one who made the term.

Removing the Limiter
In rare instances, it is possible for certain individuals to remove their limiters. Genus claims that this requires certain conditions. Becoming a Mysterious Being is not the same as removing one's limiter: It is a mutation triggered by negative emotions, experimentation, or the injection of Monster Cells, and the individual is reborn as a new being with a new limiter. Abnormally powerful humans such as the heroes always had the potential to become extraordinarily strong, and those who modify their bodies must modify again in order to improve beyond their natural limitations; they also have not broken their limiters. Saitama, a man who has supposedly broken his limiter, was an ordinary person with no talents or potential, but through effort alone, he evolved beyond his natural limits and achieved power that Genus is unable to even fathom. However, he became bald.

Gyoro Gyoro's Method
I did everything I could to figure out a way to break through the limit of growth...monster cannibalism, blood and cell transfusion, breeding. Experiencing different types of pain and stress. Increasing their level of anger and hatred. Everything I could think of, I've tried. During this process...I found out that it is possible to reach the next stage if one conquers death. And if the process is repeated, you end up with explosive growth.Gyoro Gyoro seeks to create the "strongest monster" by breaking its limiter. After many failed experiments, she managed to find a way to seemingly do so: overload the body and spirit of a human by pushing him or her to the brink of death repeatedly, with the intensity tailored to the individual. Despite how simple it sounds, death can occur at any time in the process, and even if one survives, they might lose their minds, rendering the monster "mediocre." Although Gyoro Gyoro had made several monsters that grew fairly powerful this way, they all eventually died before reaching Disaster Level Dragon. She does claim to have one success though: Orochi, the Monster King.

She attempted to repeat the process to Garou by sending a bloodthirsty Royal Ripper and Bug God to tail him, knowing that they will fight him to the brink of death. These conditions, alongside the many other near-death battles Garou has fought, allowed him to break his limiter and become an immensely powerful monster that continuously gets stronger, just like Orochi. When he was breaking his limiter, Garou mentioned how his body felt light as if his "spirit" was removed from his body.

During Garou's battle with Superalloy Darkshine, there is narration stating that "Garou's limiter [...] seems to be breaking!" He experienced explosive growth and quickly increased his speed, strength, and other abilities, even while in the midst of battle; he could even adapt to psychic abilities. When he punched Golden Sperm, the latter remarked that Garou's punch was many levels higher than what humans are normally capable of. In the end, however, Garou began running out of fumes and lost the power he had gained. Whether this means that he had failed to completely remove his limiter, that his limiter was placed back on, or some other reason remains unclear.

Trivia

 * It is possible that the "God" Dr. Genus refers to is the same being that Homeless Emperor calls God.
 * Unlike Saitama, individuals who supposedly broke their limiters via Gyoro Gyoro's method become monsters, instead of bald.
 * However, in the case of Garou, it appears to be different somehow. He did not abandon his humanity and Saitama did not consider him to be an actual Mysterious Being, accusing him of being just a human in a monster costume. Also, unlike any Mysterious Being so far, Garou's transformation managed to revert. Saitama is also very aware that humans are capable of turning into Mysterious Beings, so the fact that he saw Garou as a human after seeing his monster form hints a possible difference between Garou's transformation and a regular Mysterious Being transformation.
 * Gyoro Gyoro's method only works successfully on certain people. She evaluated Garou and could tell that he had the potential to be another "success," like Orochi. According to Genus at least, Saitama had "no potential" prior to breaking his limiter.
 * Dr. Genus did not know Saitama back then, however, so there is no way for him to truly know if Saitama had no potential. Saitama managed to kill Crablante, a Tiger-level monster that normally requires an A-Class hero to defeat, without any training or experience or weapons.