Are SSH keys and PGP keys the same thing?
I have a 3072 bit RSA key that I generated for use with SSH. Can this key-pair be used with PGP/GPG, or do I need to generate a new pair of keys separately for use in email encryption?
Are the two interchangeable, or is there a difference between the two?
Even if they were, it's generally a bad idea to re-use the same key across multiple contexts.
Both PGP and SSH support RSA public/private key pairs, so presumably it is possible.
Yes, the same RSA key pair can be used for both (Open)SSL and OpenPGP/GnuPG.
The monkeysphere project contains a tool to convert RSA keys in PEM format to the one defined by OpenPGP,
pem2openpgp. For converting the SSH key pair into the PEM format, there already is a comprehensive answer in Converting keys between openssl and openssh.