GPG is the open-source implementation of PGP — providing email encryption, file encryption, and digital signatures using a web-of-trust model instead of PKI CAs.
GPG uses web-of-trust: users vouch for each other's keys. PGP and GPG are interoperable. Used for secure email, signing Git commits, and verifying software downloads. Less common in enterprise (SMIME preferred) but standard in open source communities.