Thanks @Lukas and I totally understand. I know a few others are going to land on this thread, however, as it’s pretty much the first thing that comes up in Google from recent mentions of this.
For me, I was able to get it fixed by re-creating the service file which is missing in the latest Kali. You can add the following contents in the /usr/lib/systemd/system/greenbone-security-assistant.service file:
[Unit]
Description=Greenbone Security Assistant (gsad)
Documentation=man:gsad(8) https://www.greenbone.net
After=network.target gvmd.service
Wants=gvmd.service
[Service]
Type=forking
User=_gvm
Group=_gvm
ExecStart=/usr/sbin/gsad --listen=127.0.0.1 --port=9392
Restart=always
TimeoutStopSec=10
[Install]
WantedBy=multi-user.target
Alias=gsad.service
Hope this helps others who will run into these same issue.