OpenPGP also hashes the package content's, but additionally cryptographically signs the hash.
A simply hash sum only allows to detect transmission problems. It does not allow to detect attacks, at least not as long as the hash is not verified through some secure channel.
Given the signer's key was validated and is trusted, OpenPGP provides such a secure channel and allows to verify the package's origin in addition to detecting transmission errors. There are different ways to validate an OpenPGP key, but simply fetching a key from a key server should never be considered one.