Can pgp universal server using Global Catalog, or only LDAP?
The PGP server only uses Ldap to query your Active Directory for user seaches. If you have more than one Directory this can be added in Ldap.
The answer is yes, you can use GC(3268) and Secure GC(3269).
Keep in mind if you work with AD Groups to make them Universal (instead of global) so items in this groups will be found (replicated trough AD).
And of course you could add multiple GC's.