Hi,
please note, that you are using an outdated version. The current GSA version is 9.0, as you can see here
I’m not familiar with the installation on Kali Linux, as this is a uncoordinated integration we do not support here. You may want to contact your package maintainer here.
However, about the admin user: IIRC you can add an admin user for login with following command:
gvmd --create-user=<username> --password=<password>