ParticleGPU Custom bounds?

Hi all,

I’m very new to TD and am building a project for a 5 sided space (Floor + walls).
I have a particleGPU TOP mapped on the floor and working quite nicely with OpticalFlow.
Now I want to extend this to also move across the walls and ideally fall back down with some sort of gravity force.

I have an alpha mask version of the space and the format the media server take, can I use this mask to create custom bounds for the particles to flow in?
And how would I create “localised” gravity so it only affects the particles on the walls?

Thanks!