I found some history on white gloves, although not specific to military uniforms. In a book entitled Death and Burial, the color white (and by extension the white gloves) are associated with innocence; in the case of death and ceremony, this innocence takes the form of strangers who represent the world at large. It seems that historically the undertaker's men and the pallbearers were strangers to the bereaved family and represented the respect offered to the Dead by the rest of the world. All undertakers and pallbearers wore white gloves for the ceremony; those gloves were then interred with the Deceased. In the military, honor guards were specific distinctive uniform parts, with white gloves being part of the uniform. Thus, the honor guard serves the same purpose as the undertakers/pallbearers did - representing the world at large. Not a great answer, but the best I could find.