Once you're finished, click on Layer->Mask->Apply Layer Mask.Add a grayscale copy of the layer or a blank image, whatever fits your needs.Right click on the layer you want to add an alpha channel.Therefore, on the image you're already working, go to the Layer window (if you don't have it, menu Windows->Dockable Windows->Layers): It's like an alpha channel but with a different name BUT gimp doesn't see it as an alpha channel until you apply it.
Use the code from previous section to combine the textures into Diffuse_specular, Normal_height textures.
PIXPLANT PLUGIN GENERATOR
The default material generator of the Ogre Terrain Component uses two textures per layer: