Somehow a gateway change locked me out of my OPNsense GUI. I still have the Anti-Lockout Rule enabled in the firewall so this shouldn't have been possible, but alas...
While I didn't have GUI access, I could still SSH. It turns out, OPNsense's main configuration file is /conf/config.xml.
So, first I backed up the configuration
cp /conf/config.xml /conf/config.xml.bak.
Then I opened it in vi to find my recent changes (I had enabled a route and intervace named OpenVPN)
After reversing my edits I saved the file
:wq and ran
reboot - I was back in!