Several parts of GearCity require .material files. Material files are text files which the game engine uses to load and name textures. The material editor allows you to easily create basic Ogre3d materials without the need to manually write files or use a 3d modeling tool.
The Editor can be opened in any parts of the mod tools that require a .material file. You can use other external tools to make these files as well. Materials have two parts. The .material file and the texture (.dds image). The .material file contains the Name of the material and the file name of the texture image. The texture must be a .dds file. See for information on DDS textures .
The following picture will break down the various parts of the Material Editor.
Video Examples Coming Eventually.
Most of the time this is caused by missing texture files or material file. Did you include the files in a zip with other game artwork files? Check the log file to see which material or texture file is missing.