What are the critical files I need to backup from gpg? I guess my private key would qualify of course... but what else?
The most critical are your secret/private keys:
gpg --export-secret-keys > secret-backup.gpg
secret-backup.gpg is then the file to keep safe.
Otherwise the ~/.gnupg/ directory contain all private and public keys(secring.gpg and pubring.gpg respectively) as well as configuration and trustdb which could be convenient to have stored.
The easiest way would be to grab the entire GnuPG directory - usually
~/.gnupg/, it contains all private keys you have, as well as the public keyring and other useful data (trustdb, etc.)
You definitely want to backup your private key and the revocation file you created.
You may also want to back up any keys you've signed or ones you don't feel like re-downloading off the key servers.
At a minimum, all you need is your complete key.