Just looking in my own collection on Numista, I can see that while some pages of coins that do bear a Gregorian date that for some reason have the Gregorian date in brackets (e.g. https://en.numista.com/catalogue/pieces5592.html),
there are also those without a Gregorian date that do not list the dates in Islamic calendar years, and do not have the Gregorian years in brackets (e.g. https://en.numista.com/catalogue/pieces22003.html).
As far as I'm aware, Numista policy is to list dates in whatever calendar is on the coin itself, and have Gregorian dates if it is a non-Gregorian calendar or ND issue in brackets.