No documentation exists. That use case is unsupported, because there is no maintainer who supports that use case.
Slightly improved the error text in git:
(Underline added to match your case.)
[u]uwt wrapper could not determine if this is Whonix-Workstation or Whonix-Gateway.[/u]
Variable uwtwrapper_gateway_ip does not exist early enough in /etc/uwt.d.
[u]If you are using a Custom-Workstation, please consider installing the
anon-gw-base-files or anon-ws-base-files package.[/u]
If you are not using a Custom-Workstation: please report this bug!