I think it's time to start the process from scratch. Not the height map in general, but your check list.
Close gimp, close everything down so you can start again with a clearer mind and view.
Try this checklist:
Open GIMP and make a new canvas
Make sure sure your canvas is 513x513 pixels (other than 1025x1025, 513x513 only works- 512x512 is for presets)
Greyscale
White Background
8-bit (you can try 16-bit later if this works)
Open up your height map then copy and paste it into the new canvas you made
Merge all layers
Alpha Channel off
Save as a PNG file to Local Disk (C:) \\ Feral Heart \\ Media \\ Terrains
Make sure you don't save it with opacity (if I remember GIMP correctly)
Bless xD
Glad the hardship is over