Hi Jo,
see gpgme bindings at https://wiki.gnupg.org/APIs
otherwise I believe that windows crypto api should give you low level
functions for dealing with the hybrid decryption (though it may take a bit of code to get revocation and other PKIX features working).
as this is a wiki, some entries may be out of date.
I’ve at least found a new link to the gpgme-sharp source code.
I cannot say if this is a good base for a library or not.
One additional way is to describe the task you want to solve
and ask on one of the mailinglists for hints.