First off, in the Gran Turismo seies of games there are usually no traditional type codes, like press x,o,circle for infinite credits, but there are loads of things you can unlock for completing various tasks. Here are a shed load...

you can get the following cars by completing the corresponding tasks

Aston Martin Vanquish
Win the British GT Car Cup in the Professional League.

Audi S4
Win the Tourist Trophy in the Amateur League.

Audi TT 1.8T Quattro
Win the Tourist Trophy Audi TT Race in the Beginner League.

Celica TRD Sports M
Win the FF Challenge in the Amateur League.

Chevrolet Camaro SS
Win the Stars and Stripes Grand Championship in the Beginner League.

Citroen Xsara Rally Car
Win the Super Special Route 5 Wet in Rally.

Daihatsu Mira TR XX Avanzato R
Win the Turbo Race of Turbo Sports in the Beginner League.

Ford Escort Rally Car
Win the Tahiti Maze in Rally.

Ford Focus Rally Car
Win the Smokey Mountain Rally in Rally.

Honda Civic SiR-II
Win the Type R Meeting in the Beginner League.

Honda CRX Del Sol SiR
Win the NA Race of NA Sports in the Beginner League.

Honda NSX Type S Zero
Win the MR Challenge in the Amateur League.

Honda NSX Type-R
Win the Type R Meeting in the Amateur League.

Mazda 787B
Win the Like the Wind in the Professional League.

Mazda Eunos Roadster
Win the Clubman Cup in the Beginner League.

Mazda Miata Roadster RS
Win the Spider/Roadster Cup in the Beginner League.

Mazda RX8
Win the NA Race of NA Sports in the Amateur League.

Mazda Savanna RX7 Infini III
Win the 80's Sports Car Cup in the Beginner League.

Mercedes Benz CLK Touring Car (D2 AMG Mercedes)
Win the Legend of Silver Arrow in the Amateur League.

Mercedes SLK 230 Kompressor
Win the Legend of Silver Arrow Mercedes Benz in the Beginner League.

Mines Lancer Evolution VI GSR
Win the Turbo - Race of Turbo Sports in the Amateur League.

Mini Cooper 1.3i
Win the Lightweight K Cup in the Beginner League.

Mitsubishi Lancer Evolution IV GSR
Win the Evolution Meeting in the Beginner League.

Mitsubishi Lancer Evolution V GSR
Win all Golds in the Beginner League.

Mitsubishi Lancer Evolution VI Rally Car
Win the Evolution Meeting in the Amateur League.

Mitsubishi Lancer Evolution VI Rally Car
Win the Smokey Mountain II in Rally.

Mitsubishi Lancer Evolution VII GSR
Win the 4WD Challenge in the Amateur League.

Mitsubishi Lancer Evolution VII Rally Car Prototype
Win the 4WD Challenge in the Professional League.

Mitsubishi Lancer Evolution VII Rally Car Prototype
Win the Swiss Alps II in Rally.

Nismo 400R
Win the Race of Red Emblem in the Amateur League.

Nismo Skyline GT-R S-tune
Win the 80s Sports Car Cup in the Amateur League.

Nissan Silvia K's 1800cc
Win the FR Challenge in the Beginner League.

Peugeot 206 Rally Car
Win the Swiss Alps in Rally.

Shelby Cobra
Win the Spider & Roadster Championship in the Professional League.

Spoon Sports S2000 Race Car
Win the Stars & Stripes Grand Championship in the Amateur League.

Subaru Impreza Rally Car Prototype
Win the Super Special Route 5 Wet II in Rally.

Subaru Legacy Blitzer B4
Win the Boxer Spirit in the Amateur League.

Suzuki Alto Works Suzuki Sports Limited
Win the 4WD Challenge in the Beginner League.

Suzuki Escudo Pikes Peak Version
Win all Golds in Rally.

Team ORECA Dodge Viper GTSR
Win all Golds in the Amateur League.

Tommy Kaira ZZII
Win the MR Challenge in the Professional League.

Toyota Celica Rally Car
Win the Tahiti Challenge of Rally in Rally.

Toyota Celica SS-II
Win the Altezza Race in the Beginner League.

Toyota Corolla Rally Car
Win the Tahiti Challenge of Rally II in Rally.

Toyota MR-S S Edition
Win the MR Challenge in the Beginner League.

Toyota Sprinter Trueno GT-Apex (AE-86 Type I)
Win the Sunday Cup in the Beginner League.

Toyota Sprinter Trueno GT-Apex Shigeno Version
Win the FR Challenge in the Amateur League.

Toyota Vitz RS 1.5
Win the FF Challenge in the Beginner League.

Toyota Vitz RS 1.5
Win the Vitz Race in the Beginner League.

Volkswagen New Beetle RSi
Win the Beetle Cup in the Beginner League.

Cars can also be unlocked by winning the different championships... So go for it!!!