How to Make a Geometry Node Apply a Blur Effect to a Specific Grease Pencil Layer?

I thought I could do it like in the image above, but I can’t figure out how to implement it. The join geometry node doesn’t work well with grease pencil.

I want to apply a blur effect to specific layers, so I feel like the geometry node might be the solution. The blur effect should take parameters like samples and size, duplicate specific layers the number of sample times, move them slightly, and lower their opacities.

I don’t want to separate the grease pencil object just to apply effects to specific layers.
Can someone help me with this?