The copyright attached to the gazetteer is described here.

Basically, what is means is that you can’t use it for commercial purposes or reproduce it an altered form without my consent. Otherwise you can do what you like with it. The same goes for the scruffy hand-drawn map. The pretty map is a somewhat different kettle of fish, as that belongs to Gollancz and/or the artist. I imagine pretty much the same conditions would apply. So no printing it off and selling it.

Using the gazetteer as a background for your RPG group doesn’t count as altering it. I’d be flattered.

And credit where credit is due. Several places in the real world were used as source material for some of the places in the gazetteer (bits of the Taj Mahal, for example, appear in the article on the Palace of Paths, the Aardish Caves, The Fortress of Watchfulness and the Veid Palace). have fun spotting them. Wikipedia was a frequent source of reference material.