I posted this in another thread, but it occurred to me that every skinner could use this information.
Gloss maps have several color channels, they are used as follows:
red = specular sharpness, how spread out the highlight is, metal should have a narrow / low / dark value, plastic or shiny wood a higher value.
green = specular level, how shiny the highlight is, higher is brighter. Metal should have a high value, shiny plastic less, matte (bark, cotton) should be black on the green channel.
blue = cloth dirt mask, gets combined with info from the game about dirtiness for the specified model and the default dirty map (color and alpha), the higher, the dirtier it can get.
alpha = faction color strength, more alpha means the faction color gets more priority over the diffuse color.