Outside of his Change of Hearts appearance, I have never heard any pre G Unit Game material. I am not sure any exists. What I remember hearing is Game was a dude that could out rap anyone but couldn't make actual songs. He was sitting stagnant on Aftermath so Iovine decided to hook him up with 50 and G-Unit as a way to generate a buzz, since they where the hottest thing in music at the moment. This worked and Games stock skyrocketed, so they decided to let Game run with it and continue to cash in off the G Unit craze. But Game still couldn't write hit songs, so as a way to repay Dre and Em, 50 gave him songs that he had intended to use for the Massacre.
Now, this is just what I gathered through different interviews and other media, so it may not all be true. But it seems like the most likely scenario.