That's a good idea, TakeMangaka. I found a tutorial on a PDN user's website (I forget who it is. I'm sorry I can't give them credit.) on how to make gradients. It's not exactly perfect, but they created blocks of colors and used a blur effect (I forget which one, probably just a Gaussian) to make the colors look blended together like a gradient.
*Thinks to self: I wanna say it was one of CMD's tutorials, but I can't quite put a finger on it.*