How do you get the ballad of gay tony bonus download for ps3 for free?

I'm sorry to say that is not possible. However, you could buy the GTA: Episodes of Liberty City and get both The Lost and Damned and The Ballad of Gay Tony.

An alternate way would buy to get on your console marketplace (Steam, Xbox Marketplace, PSN Store) and buy it from there. I don't know much about other consoles, but on the PS3, you can purchase the GTA Episodes for $19.99