The Real World Theory is a possible myth and an Easter egg in the Grand Theft Auto franchise (excluding the first two games and the first game's expansion packs).
Description[]
The Real World Theory, as it name suggests, posits that the world of Grand Theft Auto co-exists with the real world, meaning that every fictional, parody versions of real life locations, people, brands, etc. live in the same universe, and the same Earth, as their real counterparts. This, of course, comes with several major differences from the actual real life world, such as the United States being geographically larger and having one hundred states as opposed to fifty, and having a population of six hundred million instead of three hundred.
Evidence[]
Across most games, real life singers are mentioned on the radio, with their songs on rotation.
In GTA III, Ray Machowski sends Claude a flyer that includes a picture which reads "See you in Miami." The same picture can be seen on billboards in the city.
In Vice City, Hollywood (the real counterpart of Vinewood) is mentioned in some radio songs.
In San Andreas, characters sometimes mention California and Los Angeles. It is said on the radio after the mission "The Da Nang Thang" that Madd Dogg missed a concert in LA.
In GTA IV and it's expansion packs, sometimes while listening to the in-game radio, the player will hear someone mention a Californian accent, implying the state's existence. One character states that they're from Florida, despite GTA VI being confirmed to take place in a parody version known as Leonida.
The evidence in GTA V is larger. At the gas station across from Trevor's trailer, one can find a group of real life state license plates. The states included are Washington, Colorado, Michigan, Ohio, Maryland, and California (included twice, with different designs). Jimmy De Santa also mentions Batman at one point in the game.
On the in-game radio, there are songs called "Los Angeles" and "California Girls", respectfully.