@Tim Edwards may know, but for the digital GPIO I would expect no less than 1-2 pF in terms of on-chip contribution.
@Anchit Proch, by analog GPIO you mean the bare analog pads in Caravan? Excluding your own ESD that you will have to add, I think the bare pad should be no more than 100 fF or so. Most of the cap will come from the package and whatever you connect on the outside (hard to keep the sum of these below ~1pF).