Would think they are Marant dicker boots, which have been available in varying shades of grey/taupe. Beautiful beautiful boots, but the sole is insanely thin and soft, so would need doing before wearing and regularly thereafter if you want to do anything other than pose in them.
Can you tell I've just returned some?
Fwiw, similar boots available from Aldo, new look, Russell & Bromley and ash, among others. I have just bought the Aldo ones, think they're called Qualheim, but they are camel rather than grey.