There is a presentation[1] by grugq on how to communicate securely. Using gpg with email is hard. Anecdotally, people reply in cleartext quoting the encrypted message. If you can train and test your co-conspirators before sending them incriminating evidence about yourself, maybe it can be done. But mistakes happen so often, people who have secrets that can get them killed or jailed for a long time just switched to Signal. That's what tptacek means.

1 - https://grugq.github.io/presentations/COMSEC%20beyond%20encr...