Yes definitely.

In your xcf file you should have a number of layers, one of which contains your walls. Now what you need to do is right click that layer -> Alpha to Selection. That should now have all your walls selected. Go to Select->Grow and pick a number of pixels. This number will be the number of pixels that the grid stops at before it hits the wall - essentially your padding. Here I used 5 pixels:
padding.jpg

Now create a new layer and move it between the walls layer and the grid layer:
padding2.jpg

With this layer selected, fill your selection with white (Edit->Fill with background colour if you have white as your background colour, or ctrl-. for quick). This should give you a nice white padding around your walls:
map9.jpg

Glad it was helpful.