I have never seen an extruder/clay gun, but the concept sounds similar to a cookie press. Has anyone used a cookie press for fondant/gumpaste? I ask because I have only seen one for sale on a NZ website, and it is $95 + shipping. Cookie presses are quite a lot cheaper!
You could give it a try. My concern would simple be that the fondant is a lot stiffer than cookie dough so the cookie press might not be sturdy enough to handle it.