In the context of a fictional universe, the canon comprises those novels, stories, films, etc., that are considered to be genuine or officially sanctioned, and those events, characters, settings, etc., that are considered to have existence within the fictional universe. This is the illusion of true fiction. The word "canon" originally meant the books which the Catholic Church officially chose to be included in the Bible - by extension, it means the authoritative "holy writ" of a fictional universe.
In the Grand Theft Auto universe, the events that are considered to be canon, or to have officially happened, are only those in the games' main storylines (GTA to GTA V) and cutscenes (including The Introduction).
Any events in GTA Fan Fiction or details that change as a result of modifications are not considered to be canon, even if they do not contradict canon. Also, GTA canon does not include events that the player enacts on their own, such as killing a character (which is usually ignored by the game, or simply results in a failed mission) or dying themselves (as the game resets to the last save point).
Many universes have contradicting canon, some of which are big discrepancies (such as a death), or particularly with intricate details such as event dates, or numbers that are approximated in writing (such as numbers in a crew or army). The creators of a universe generally define what is official canon - the easiest definition is the principle of first mention (where any discrepancies are treated as errors that do not respect the original canon), whereas others prefer to be able to redefine canon in perhaps more detail later on (such as where the original film/book/series is ignored in favour of better and more thought-through details).
Any information that is non-canonical must be marked with {{unofficial}} on every page or in every non-canon section on a mostly-canon page.