Creating a gradual blur across the image

I do a lot of work that involves gradual blurs for various purposes, including simulation of depth of field under a variety of circumstances. I'm aware of all the traditional ways of doing this, including a variety of DOF simulation filters that I use, camera DOF, and using a precomped solid layer with a black/white ramp to drive the blurriness of a Compound Blur.

I find that I need to do the latter often, and its particularly irksome, especially in complicated compositions, to be forced to create ramp effects and precomps for every single gradual blur I want to create. Especially when the comps get complicated, I hate to have a ton of otherwise useless layers and precomps cluttering things.

I feel like there must be some expression that exists to vary blur-amount across the frame based on say, the Y position of a given pixel, in order to create a clean gradual blur. How would I go about doing this?