GPG: Extract private key and import on different machine
After extending the expiry date of a GPG key you might have to copy your key to another machine to use the same key there. Here is how:
Identify your private key by running gpg --list-secret-keys. You need the ID of your private key (second column)
Run this command to export your key: gpg --export-secret-keys $ID > my-private-key.asc
Copy the key to the other machine (scp is your friend)
To import the key, run gpg --import my-private-key.asc
If the key already existed on the second machine, the import will fail saying 'Key already known'. You will have to delete both the private and public key first (gpg --delete-keys and gpg --delete-secret-keys)