GM Terrain Editor 2 is a tool for generating terrain height map and texture.

You can download GM Terrain Editor 2 from here
http://gamemaker3d.com/files/GMTerrainEditor2.zip

After lunching the application, first change the terrain size to 255×255

When you finish to build your terrain, you can export the heightmap with the following button

You can find the terrain height map file in the application folder

Now you can set the heightmap.png as your terrain height map file