Which Grand Theft Auto game is described as the sixth main entry in the series?

Grand Theft Auto IV is described as the sixth main entry in the series.

Released in 2008, Grand Theft Auto IV follows Grand Theft Auto: San Andreas and precedes Grand Theft Auto V in the numbered main series. It is also described as the eleventh entry overall because the wider count includes spin-offs and other non-numbered releases.

The game introduced a new continuity often called the HD Universe. Its story follows Niko Bellic, an Eastern European immigrant who comes to Liberty City seeking a fresh start and is drawn into organized crime, family disputes, and the schemes of his cousin Roman.

Players sometimes count only the numbered titles and assume Grand Theft Auto IV is the fourth main game. That is not how the series classification works: Grand Theft Auto: Vice City and Grand Theft Auto: San Andreas are considered main entries even though their titles do not use Roman numerals. The main-entry sequence is Grand Theft Auto, Grand Theft Auto 2, Grand Theft Auto III, Vice City, San Andreas, and then Grand Theft Auto IV.