The RPM manpage says that PGP is currently the only one supported. 

"In order to use the signature feature RPM must be configured to run PGP
[or GPG], and it must be able to find a public key ring with RPM public
keys in it. 

_pgp_path is llisted
but through my testing, _gpg_path also works. 

_signature states that "The Signature type.  Right now only pgp is
supported."
My testing indicated that gpg in that field works fine. 

_pgp_name the name of the "User" whose key you wish to sign your packages. 
Through my testing, i discovered that the 'pgp_name' had to exactly match
the name of the UserID in my gpg public key.  You might want to mention
this.