The code is said to contain a list of cars, planes, helicopters, boats, bikes and trains that will appear in the upcoming game. It was posted by a user on GTAForums.
Although unconfirmed by Rockstar, Grand Theft Auto code has previously appeared in games such as Red Dead Redemption and Bully, reports IGN.
Grand Theft Auto 5 has been tipped for an October release following reports that a member of its development team let the date slip via his online CV and LinkedIn page.
BioShock Infinite's recent delay, meanwhile, was attributed to a potential Grand Theft Auto 5 release that month, according to some analysts.
Other analysts claim the open world action title will more likely release in 2013.
Grand Theft Auto 5 takes place in the city of Los Santos and its surrounding hills, countryside and beaches. Its lead character is thought to be a man named Albert De Silva, played by Ned Luke.
Rockstar is expected to announce an official release date for Grand Theft Auto 5 at E3 in June.
The list of Grand Theft Auto 5 vehicles in full:
Watch the Grand Theft Auto V announcement trailer below: