“The Gorgon,” by Tanith Lee
Appeared in Shadows 5, an anthology of quiet horror (Doubleday, October 1982, edited by Charles L. Grant); collected in The Gorgon and Other Beastly Tales (DAW Books, 1982); appeared in Nightmare Magazine (read here), February 2014 (issue 17); featured in PseudoPod 604, July 26th, 2018
8,001 words—I really thought it was less
Interesting. (Spoilers.) I wondered how this story would reveal its Medusa, and it managed to surprise me, presenting a mundane disfigurement followed by the real Medusa, the ugliness inside the protagonist himself.