Yep, gaboons are best kept on natural substrates with real leaf litter... Once in a while I will find some isopods and other assorted inverts in the cage but that is fine... Isopods are actually good to have around as they eat fecal matter and rotting vegitation.

If you get a chance, try to get a back issue of the Oct 2006 issue of REPTILES mag... There is an article written by myself that gives a detailed rundown of husbandry requirements for the Gaboon Viper... There are also other great articles on safe husbandry and other keeping issues...
It might be helpful to you...