You can use burned CD. Burn gpg ciphertext from computer with internet to CD, put CD in computer with no internet access to decrypt. That prevents plaintexts to you and private keys from being vulnerable to hackers. It is harder to do this for responses though, because you need to load public key from the machine that has internet access to a machine that you encrypt your outgoing plaintexts on that also never has internet access or contact with anything that ever will (after you start typing plaintexts on it). This is called an airgap system. What advantage does that give?