Displacement will give you the best result… But if you don’t want to use displacement you need to use Normals.
Creating good normal maps (and bump maps) is another story; And probably the source of your problem…
Note that Normals only give a different orientation to the surface, and not a height value. This makes it impossible to distiguish a sunked area from a raised one. The only thing it will show is the slope from one height to the other.
This is important because you say your Bump is a flat black/white picture; and if so, the slope between the black and the white will be almost 1 at the limits but 0 all over. Bluring it a bit can produce better results, since the slope will be distributed over a larger area.
I confess that never used Gimp plugin for creating normals, so I don’t know if there are any parameters to tweak for a better result.
Could you post the texture you’re using for creating the normals (the one you want to engrave)?