Say I want to create a gradient that only affects saturation, how do I do that? Example: I have an image that I want to hint at perspective in. I want the objects farther back in space (top) to be less saturated and those closer to the foreground (bottom) to be more saturated. I'm assuming I can do this with an adjustment layer and I've tried messing around with it, but it seems that I have to set colors to create a gradient but I don't want to affect the native colors at all, I just want to change their saturation. I'm not talking about going from B&W to color. I want to be able to set a particular level of desaturation grading to another level of saturation up to 100%.
Create a hue/saturation adjustment layer. You'll see a mask associated with the layer (thumbnail to the right of the adjustment layer icon in the layers panel). Use the gradient tool on that layer mask.