Almost any color can match a gray skirt, because gray is a neutral color. Since it is not on the color wheel, there is no particular color or set of colors that it needs to be paired with or cannot be paired with. Shades of dark gray typically go better with brighter colors and shades of light gray look better with darker colors for a little bit of a contrast.
When wearing a gray skirt, black tights typically match best since they are also neutral and make room for more versatility in shirt and shoe color. Neutrals can be paired with other neutrals, as in the case of black and gray, but they are ideal for mixing a variety of colors including burgundy, purples, and blues.