As noted, the playlists are compiled from music already available on these services. There are some songs that don't yet exist on either Spotify or iTunes. If you are one of those gamers who enjoyed the commercials more than the actual tracks, be sure to check out The Advertising Council repository. It's a collection of all of the radio advertisements in the GTA franchise.
For the full list of Grand Theft Auto radio station playlists, check out the official Rockstar site.