Gradient Bump in P5 is NOT a bump creation process at all - it is a "layer" which is luminance multiplied with the underlying material. In other words, it mixes the luminance values in the *.bum map with the luminance of the texture, creating the APPEARANCE of highlights and shadows but they are NOT responsive to your scene lighting. If you look at a *.BUM file you will note that it is just an "EMBOSSED" image like you can make in most paint programs. Poser uses that luminance information and discards the color information to create the illusion of bumps. This is not the same thing as real bump mapping which IS responsive to your scene lighting. Knowing this you can do some interesting things with the Gradient Bump node in P5 (such as apply body hair, skin blotches, freckles, etc to any texture (provided you only need the luminance channel to create your effect because color information in this node is ignored). So if you want bump mapping from an image that is NOT a pre-embossed *.BUM file, you MUST use either displacement or the regular P5 Bump Node and NOT the Gradient Bump Node.