I found that gimp had horrible memory management as soon as maps start getting large. One thing to check is the dimensions of your layers. PS only uses memory for the areas of the layers that have something in them (I think). Gimp uses memory for all pixels in a layer, even if those pixels are totally transparent. You want to make sure that any layer in gimp is as small as possible in dimensions, rather than allowing all the layers to be the size of the image. I found that helped a lot when I was using it.