I can't give a solution to this, but I do have something on the subject. First of all, if your assumption was that you can only create heightmaps in gimp, this is not true. You can use greyscale in average old Microsoft paint, but what you can not do is blur colours for smooth transitions. Gimp for me usually comes up slowly and says "not responding" before it finally realizes it is actually functional, and comes up fine. Your computer may just not be able to run it. Can you say at what point of the loading it freezes? Have you tried re-installing it?