Okay, I figured out how to get it to work.

Here's what I did:

Start with the black walls layer and:
1) Duplicate layer (Layer>Duplicate Layer) Note:You will be sitting on the duplicate layer ready to edit it.
2) Filters>Blue>Motion Blur
2.5) For motion blur settings I used length=10, angle=225
3) Select>By Color and click in the background;
4) Select>Invert (this will result in all the blurred areas being selected)
5) Fill the selection with black;
6) Filters>Blur>Gaussian Blur>
6.5) I used a horizontal and vertical blur of 70 just messing around;
7) Now Drag this layer beneath your walls layer.

The map so far (WIP):



Again, thanks Jax!!!