this is a bug not a feature. i used ricardos 2003 suzuka as a base, he obviously editet the visual properties of the run off, ie looks like tarmac but at the time physical properties werent edited, .col file i think. i did edit a few bits to make them match but obviously missed this bit.
read what lo2k wrote, he's the expert.
i understand what you want but it would require defining a new track property as mentioned above.
currently you have tarmac which obviously behaves like tarmac
and gravel which behaves like gravel, what you want is gravel that behaves like tarmac and gravel, ie hard as tarmac but sprays dust like gravel.
note this property is independent of actual texture.
perhaps you could ask madman on his forum. hes also another gp4 genius. you should try out his latest beta of tweaker, which includes slip stream effects and much better force feedback effects.