So, does your heightmap you make and try to put in the tab not work? Does it crash?
If it does, make sure it's only ONE layer, greyscale and 513x513. If you have the new version of GIMP also, then export it as a .png to the correct spot, and then save it as well in case if you lose the other one.
If you have the old version, I think you can just save it as a .png instead of exporting.