GPG decrypting should not be intrusive
As soon as you receive a gpg encrypted message, it asks for decrypting private key (and freeze gajim). If you are not present in front of the computer, gajim is frozen potentially for several hours (and will not receive new messages).
In my opinion, when receiving an encrypted message and the key is not unlocked, it should only add a button "decrypt" in conversation box, and asks for passphrase only when clicking on it.