Is this what we should use on the gateway and the workstation to keep it up to date?
sudo apt full-upgrade
And reboot I suppose, but is there more we should do to keep it up to date?
Yes.
Except if you have manually installed software (without APT) or from something like flatpak or flathub then you need to care of that too. If not then not.
And sometimes release upgrade if there’s a new major version.
Release upgrade?
Do you mean “do-release-upgrade”? I don’t think that’s a binary that exists on the whonix server?