I can see why a normal map uses the Gradient Bump input—if I read the manual right it's only used by the P4 render engine for .bum bumpmaps—but there's always been stuff out there which misuses it. I would have been tempted to change the label.

But I've never been that clear on bumpmaps. How can that single value, of the greyscale at a point in UV space, define a change in the apparent angle of a surface?

Let me know if the Poser manual ever gets past being an aide memoire for highly-trained CGI professionals.