PGG is an interface library between Emacs and various tools for secure communication. Even though Mailcrypt has similar feature, it does not deal with detached PGP messages, normally used in PGP/MIME infrastructure. This was the main reason why I wrote the new library.
Note that the PGG library is now obsolete, replaced by EasyPG. See EasyPG.
PGP/MIME is an application of MIME Object Security Services (RFC1848). The standard is documented in RFC2015.