Currently still manual tweaking is required.
a) qubes-prefs --set updatevm whonix-gw
b) Using whonix-gw as ProxyVM for TemplateVMs.
For a), to implement this, creating a uwt wrapper for yum could be sufficient.
For b), not sure yet.
Recent forum discussion:
https://www.whonix.org/forum/index.php/topic,1578.0.html
Related Qubes ticket:
https://github.com/QubesOS/qubes-issues/issues/1159