During the Middle Ages, most of the common peasants wore outfits made from undyed wool, which was a type of gray. So, it was seen as the color associated mostly with poverty.
Shades of gray are mostly identified with uncertainty or indecisiveness, indifference, neutrality, boredom, old age, humility, depression, loss, and modesty.
Gray is seen as a neutral color and as with all neutral colors, it can technically not be found on the color wheel. Your other neutral colors are white, black, and a few shades of brown.
When painting, it will be up to you to decide whether you need to use warm or cool gray colors. The decision will rely heavily on what you are painting.