If you apply a gradient to a terrain in the terrain editor and tell the terrain editor to keep the gradient, this gradient will not be saved when added to an object library, although the thumbnail will give the impression that this is occurring. If you want to add a terrain with a gradient to an object library, apply the gradient as a material from the the Material Library or your own custom material before adding the terrain to an object.
This is by default as the gradient is meant only to be a tool to aid in editing the terrain. If you want an actual gradient texture to appear on the terrain, you should apply it in the MATERIALS Library. You can choose from several gradient textures in the the 'terrain' portion of the Material Library.