So, das Problem scheint folgendes zu sein:
If you like, you can import a greyscale Heightmap created in Photoshop, or from real-world geography data and apply it to your Terrain. To do this, choose Terrain->Import Heightmap - Raw..., then select the desired RAW file. You'll then see some import settings. These will be set for you, but you have the option of changing the size of your Terrain from this dialog if you like. When you're ready, click the Import button. Once the Heightmap has been applied to the Terrain, you can edit it normally with all the Tools described above. Note that the Unity Heightmap importer can only import grayscale raw files. Thus you can't create a raw heightmap using RGB channels, you must use grayscale.
Allerdings hilft weiterlesen:
Any other heightmap editing application like Bryce, Terragen, or Photoshop can work with a Unity Heightmap at full resolution.
Terragen und PS willst du nicht kaufen aber
Bryce hat anscheinend ne free Version.
Wenn du nen Konverter für Gimp schreiben willst, dann brauchst du nen Hex-Editor und musst dir so ne Heightmap mal selbst ansehen.
Es scheint einfach eine aneinanderreihung von 16-bits zu sein, also ziemlich simples Format.
Nuff said.