If you're after a Dulux stand-in for Benjamin Moore Classic Gray, reach for Dulux Vintage Chandelier. It lands at ΔE 1.1 from the original, which is well under the threshold where most people can tell two colours apart in the same light — call it imperceptible in practice. Its LRV of 75.6 keeps it firmly in that soft, airy off-white-with-a-whisper-of-warmth territory that makes Classic Gray such a quiet workhorse.
Running neck and neck is Dulux Khaki Mists 6, also at ΔE 1.1, with an LRV of 76.4. The slightly higher LRV means it'll read a touch lighter on a really bright wall, but the difference is tiny. Honestly, between the two it comes down to which Dulux fan deck or mixing scheme your decorator already has open.
Here's the "but what about" worth raising: Classic Gray is famous for being a chameleon. It can drift mushroom, faintly green, or almost clean white depending on orientation and the light bouncing around the room. Your Dulux match will do exactly the same — matching the colour doesn't iron out that behaviour. In a north-facing room expect both to cool down and lean greyer; in warm afternoon south light they'll soften and warm up. That's the colour doing its job, not a bad match.
Don't trust a fan deck chip for a final call. Get a sample pot, paint two coats onto a piece of lining paper or an offcut of board (A2 size, not a postage stamp), and prop it against the wall. Look at it morning and evening before you commit.
One practical note: if the match matters because you're patching or extending existing Benjamin Moore work, don't switch brands mid-wall — different bases and binders can flash or sheen-shift even at ΔE 1.1. Match across brands only when you're doing a fresh, complete surface.