Gloss maps control how sharp or soft (matte) a reflection is. This allows you to add more variation to asset look based on different materials it is made of in real life. Gloss map or mask is again a grayscale texture with black and white areas for sharp and soft reflections.
In the above example that bright areas (127-255) in the gloss map are for sharper reflections while darker areas (0-127) mean soft reflections. Also notice the "noise" added to the gloss map. This is to add further detail to the engines's surface.
NOTE 1: Glass gloss maps are generally all white because they are small and use tilling. It is possible to add a few more cratches here and there when you create your own glass texture.