The difference between a digital signature and a MAC is non-repudiation. A message with a digital signature proves that only the sender could have signed the message, whereas a message with a MAC proves that either the sender or the recipient could have signed the message.

Can an OpenPGP implementation, such as GPG, be used to sign a message without providing non-repudiation?

Please note that I am referring to non-repudiation in the cryptographic sense, not in the legal sense.