If you had played Guitar Hero for iPhone/iPod Touch before, then you would have noticed that it only came pre-installed with 6 tracks. The rest of the tracks are purchasable via the in-game store which will sucks your wallet dry.
So now here's the guide to download and play those paid songs. Of course you would need to jailbreak your iDevice as we will need to transfer those song pack files via SSH.
Link to download all the song packs HERE.
Once you had downloaded the song packs on the site above, follow this guide to SSH into your iDevice. All that is remaining is to copy those song pack files that you had downloaded into /private/var/mobile/Applications/[GuitarHero] folder.
Note: One thing that you would have noticed is that you will see many folders with weird names like dd3344sdfsdf... Just go inside the folder one by one until you see there is a file named GuitarHero.app and you would have known that you are in the right folder. To make things easier, just delete the Guitar Hero app from your iDevice then resync it back via iTunes. After that go to the Application folder again and sort those folders by 'Date'. The most recent one (topmost) would be the Guitar Hero app.