The figure is likely that of George Wilson. It makes the most sense since both Gatsby's body and Wilson's body were found the next day.
George Wilson also makes sense, because he had motive to kill Gatsby. George Wilson thinks that Gatsby is the person that was driving the car that killed Myrtle. Of course Gatsby was not the one doing the driving, but he decides to take the blame for it. That way Daisy will be safe.
"Ashen" also drops a few hints to me as well. If it was George Wilson, and he was coming to kill Gatsby and himself, it makes sense that he would be nervous or anxious or scared. All of those would cause a person to look pale (ashen). I think Fitzgerald is dropping a bigger hint though than simply the color of skin. George and Myrtle lived in the "Valley of Ashes." That would make George and Mrytle two of the "Ashen People." Gatsby is "Westeggen." An "ashen, fantastic figure gliding toward him" seems to describe George Wilson to me in a variety of ways.