What you need is a COM or ActiveX component for PGP which decrypts a PGP message. There appear to be a few about, for example this one (that's not a recommendation, BTW!)
Install it on your PC, and then in Excel VB windows add a project reference to it via the Tools > References menu: find the PGP component in the list and select it. You can then use the objects it exposes in VBA code - you'll have to look at any documentation it has for the COM objects it exposes and how to use them.
Cheers -