In addition at least the systemd file posted in Greenbone-security-assistant.service missing - #5 by altjx - Greenbone Community Edition - Greenbone Community Forum has the user User=_gvm defined. As this is a non-root user the user isn’t allowed to bind on ports < 1024 including port 80 used for the redirection from 80 to 9392 which is causing this failure.
In 0007617: Greenbone-security-assistance no longer working - Kali Linux Bug Tracker the following was also mentioned:
I think you will have to remove the file you created /lib/systemd/system/greenbone-security-assistant.service.
As Kali seems to have fixed the missing gsad.service systemd file recently i would follow the suggestion above and see if the problem is already solved in the newer systemd file of Kali.