PGP and GPG

Difference Between PGP and GPG
http://askubuntu.com/questions/186805/difference-between-pgp-and-gpg

Difference Between PGP and GPG
http://www.differencebetween.net/technology/software-technology/difference-between-pgp-and-gpg/

openpgp, pgp, and gpg: what is the difference?
http://blog.goanywheremft.com/2013/07/18/openpgp-pgp-gpg-difference/