Gosh, I like this movie. It feels incredibly atmospheric, unlike a lot of Hammer films. The sets, in particular, are incredible. The house the Heitzes lease is superbly well-designed for a small soundstage: A vast interior room which access on one side to a front door and patio (with room for a mob). And a staircase, leading down to a broken fountain and another patio. The set gives some great atmospherics, with dead leaves drifting around on the wind and in the water, and plenty of space for Paul Heitz to see the Gorgon in the water and stagger artistically up the stairs to escape.
Like a lot of 1960s and 1970s horror films, The Gorgon has an ultra-conservative morality tale. In this case, artist Bruno Heitz is a "libertine" (a great line in the film!), having sex with local women. Gasp! Sex! He pays the price for it (as does she). When his father arrives in town to discover the truth about his son's untimely death, his arrogance, anger, and aggressiveness result in his death, too.
But unlike your typical horror film (The Hills Have Eyes, Last House on the Left, Halloween), The Gorgon doesn't come through in the end. The Gorgon is evil, and the deaths of Bruno Heitz and Prof. Jules Heitz are seen by the viewer to be wrong. The moral avenger (the Gorgon) is defeated for having committed her various atrocities.
I think I like this film a great deal, too, because it's one of the few times in which Peter Cushing gets to play a romantic lead. Here, he's fallen in love with the beautiful redhead Barbara Shelley. She really was a striking woman, and her ethereal beauty was never on better display.
Yes, the snake-hair effects are absolutely stupid, even for 1964. So what? Up to that point, this is a superb film.