I don't have the GM's guide but had a quick look at one in the bookshop today. The map styles don't look that difficult to recreate. I would guess they were probably done in Photoshop or Gimp. You could probably also use a reasonable vector drawing program to create them as well, such as inkscape, Xara or Drawplus.

The key would be to find similar textures, and there are a lot of stone like textures out there on the web which are near enough.

Cheers

Ravs