John Wayne Gacy, alias Pogo the Clown was born in 1942, Chicago, Illinois and was the son of John Stanley Gacy, an ungrateful and manipulative father. Gacy was beaten violently by his father throughout his childhood and suffered brutally, his father would beat him for apparently being slightly overweight and different from other ordinary kids. However, Gacy was loved by his mother and siblings alike, a fact which the father had a dislike for. Despite the love he got from his siblings and mother, his father's hatred and violence developed an internal dark side inside Gacy, which later on proved to be fatal, making him America's most chillings killers of all time. During adulthood, Gacy went on to murder 33 innocent persons, most of whom were young teenagers. He was charged on various assaults and was executed to death by strangulation, a death much painless in contrast to the painful deaths he put his victims to.
The fact that makes Gacy a horror figure in the world of myths and killers isn't only his crimes but his clown persona called Pogo the Clown. Gacy worked for children events to promote donations and various activities done for children, he would dress himself as a clown during these events. According to a psychiatric perspective, the clown was a mask that Gacy wore, to hide his dark side and avoid any suspicions that could possibly perceive him as a criminal. This is supported by a number of paintings he made during his time in prison. He also made some bizarre artwork about his clown and would often paint skulls in place of his head along with his clown dress, this was described to be a painting of the darkness that dwelled within him and was unleashed to the misery of other innocent victims that were put to death unfairly because of Gacy's evil psychological thoughts.
Basis of Existence in Grand Theft Auto: Vice City
Pogo the Monkey
Pogo the Monkey is the origin of the Pogo the Clown myth in Grand Theft Auto: Vice City. Pogo the Monkey is an arcade game based on a primate as the protagonist. The objective of the game saves Pogo from an evil scientist wanting to experiment on Pogo. Pogo's got springs instead of legs due to a genetic alteration experiment by the sadistic scientist.
Except for the name, more similarities between Pogo the Monkey and Pogo the Clown haven't been found but it is widely accepted as an Easter Egg to the clown since Pogo the Clown was a killer topic in the 1980s.
Maude Hanson's Beliefs
Maude Hanson's character is believed to reference turning points that occurred in Gacy's life. Maude Hanson, during her only cutscene, expresses magnitudes of hate towards young innocent children. This could be a reference to Gacy's father who tortured him during his lifetime. In one of her lines, Maude Hanson speaks, "Mommy doesn't love you", this could again be a reference to the hate stored in Gacy's father for the loving relationship between Gacy and his mother.
|“||Were you now, or at any time, a child?||”|
According to Gacy's father, John Stanley Gacy, his son's constant love and support from his mother and Gacy's worldly shortcoming could end up making him unconventional one day. Although, Gacy didn't end up on the streets in the coming years, contrary to what his father thought but Gacy probably could never forget about his father's unforgettable torture.
Ice Cream Factory
Maude Hanson's Ice Cream Business in the game is a front for selling drugs. According to Maude Hanson, she pretends to make children happy so that she can't be suspected for other activities.
This is a hint towards Gacy's famous clown appearance called Pogo the Clown, where he would dress as a clown to Children events, this gesture, helped Gacy's image in the public eye, portraying himself a kind, innocent and child loving person who would never commit any hideous crime. However, this was only a technique to not ire suspicion over his activities involving murders and other charges. 
John Wayne Gacy, according to police reports, dumped his victim's bodies in his house's basement. During summer, Gacy's mother-in-law would sometimes smell foul in the house, however, she never got to know about the bodies at that time. Later on, when Gacy was accused of the crimes, police uncovered the victims' bodies from the basement, many in the unrecognizable form. Similar to Gacy's horror basement, there is an unused and hidden room in the Ice Cream Factory, it could be possible that Rockstar wanted to create a reference and possible storyline but dropped the idea due to the horror content or sophisticated storyline concerns.
Ice Cream Man
The Ice Cream Man reported to wander the streets of Vice Point is based on an unknown character that drives the Ice Cream Van, using the van to sell ice cream as a front to in fact distribute illegal substances such as drugs. The Ice Cream Man myth could be a nod to John Wayne's life since one of his artworks was named “33 Flavors Clown”, a reference to his association with an ice cream chain, this was from the time when he dressed as a clown to events. 
Due to Gacy's connection and history with vans, he is reported to be linked with the Black Pony van in the game. The myth is based on the interconnection of vans and killer clowns.
Some body bags are kept besides the cherry popper ice cream for unknown reasons. These body bags could be placed as a hint towards the body bags that were a subject of the investigation inside Gacy's home. John Wayne Gacy's neighbors would often notice suspicious behavior, at times they saw Gacy removing body (trash) bags but due to Gacy's honest reputation for his children' and community works, no one believed that these activities were related to crimes in any way possible. 
|“||The only thing they can get me for is running a funeral parlor without a license.||”|
–John Wayne Gacy
John Wayne Gacy also worked as a mortuary assistant during his lifetime. Gacy once confessed that he slept in a coffin already occupied by a corpse, only to move out of it after a while, exhausted and scared. Funeraria Romero, the undertaker store (in the same area the Ice Cream Factory is), is rumored to sell corpses to Carnicero Romero, it's sister shop, a storyline much similar to Gacy's.
Pennywise was originally based on John Wayne Gacy and the acts of Pennywise are a symbolization of what Gacy felt while committing crimes. The relation between Pennywise and John Wayne Gacy has become analogous over the years, with writers describing Gacy as Pennywise of olden times. Gacy was also accused of murdering clown's innocence, transforming clowns into a suspicious and dark character and Pennywise could've been a continuation of the innocence's eradication over the years. After Gacy's crimes, many others followed his path and committed dark crimes while dressed up as clowns. Pennywise is also rumored to exist in GTA Vice City.
The year 1968, was a haunting year for John Wayne Gacy. Daniel Vorhees, a teenage boy, accused Gacy of various crimes and a result Gacy was charged in court. Due to the backlash, Gacy bribed Russell Schroeder to attack Vorhees but failed, only to further prove him guilty. Gacy was then imprisoned for 10 years, after the hearing, Gacy also got divorced and was separated from his children forever. Since the year 1968 contrasts with 1986 (year of Vice City's setting) in numbers, this is assumed to be a vague reference, however, it is unlikely.
Despite the copious amount of references included in the game, the clown's existence is missing from a technical perspective. After years of internal file searches, no files indicate Pogo's presence, let alone prove it. It is very feasible that various myths and characters in the game are a reflection of Gacy's dark side and could symbolize his dark persona.