Grand Theft Wiki:Canon
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 1 to GTA IV) 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. 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).