Quote - Gradient Bump is intended to take the old Poser 3 / 4 style .BUM maps. I don't believe these are quite the same as normal maps, since normal mapping requires three colour channels and .BUMs have only two.

The blue channel in a normal map is redundant. Since normals are 'normalized' vectors of a length of 1, you can derive the absolute value of the third coordinate from the other two, and it is positive by definition. So blue = sqrt(1 - redred + greengreen). The reason the third coordinate is saved in a normal map is because calculating it would be too expensive for real-time use like in games (which is the main application for normal maps - if you do quality rendering, you'll get much better results from displacement maps).

The main difference between normal maps and gradient maps is how they're being used - gradient bumps are being added to the existing surface normal where normal maps completely replace them.