As mentioned in the referenced older topic both parameters needs to be passed as startup-parameters to the gvmd daemon (they won’t do anything if entered into a terminal).
How this can be done heavily depends on how you’re starting gvmd. But on Kali (when using the packages provided by Kali) this is should be the systemd unit / service file of the manager daemon (gvmd.service and the ExecStart line within it).
If you need further support with the configuration on the systemd files it is suggested to get in touch with the Kali community.
Otherwise we would need to hope that a community user familiar with Kali could give some additional hints / background info.
This is a great question and the answer is very helpful as well. It would be amazing if GVM supported a user interface for configuring email alerts / notifications and changing settings in this way! Nessus as this feature built in.