This is an uncharacteristic film for George Cukor, slipping sideways into Hitchcock turf for this period. Psychological suspense was never more focused, and less distracted, than you'll find in Gaslight however. You might find the plot too linear, to predictable overall, to be blown away, but in fact that's partly why the suspense works. As with great Hitchcock, you have a sense of where you going, and you want to stop it.
So we have Charles Boyer, smarmy, deceptive, and ultimately evil, leading his new wife down a path of mental anguish and, he hopes, madness. The wife is played with usual high stakes perfection by Ingrid Bergman (between her stunning roles in Casablanca and Spellbound). Cukor gets the most of her excesses, and her nuances. Boyer is more nuance, and is a perfect match. The movie is really about their back and forth, with Joseph Cotten making his appearance as a necessary line of safety and hope because we can't stand to see the woman go down without a fight.
Most of the film occurs in an old, lavishly decorated house, and the lights and camera-work are dreamy, dripping in rim light and shadow, in odd angles and closeups of their faces. It's quite an involving experience, and because you are limited to mostly these two characters, you get very intimate with them. Yes, the two maids are perfect, including a sassy Angela Lansbury in her first movie role. And the cop, too, is a classic bobby, handsome and cooperative.
The plot, alas, is the one weakness here. The man's obsession with gems is fair enough, but when we finally get to the attic, after many months of him being there searching for them, it's as if he's up there for the first time, opening drawers with cobwebs on them, scattering through drawers like a thief with five minutes and no more. It just undermines the whole premise of a man resolutely devoting his whole devious, murderous life to this one goal.
So forget the plot, exactly. It's a MacGuffin. The real movie is in the acting, the characters in their personal wringings out, and in how beautifully it is done.