By default, Greenbone OpenVAS on BackLinux v3.13 listens on port 9293 on the loopback address (127.0.0.1).
To change this, edit the /etc/default/greenbone-security-assistant and change the GSA_ADDRESS and GSA_PORT lines.
Changing these to 0.0.0.0 and 443 respectively, will make the GUI available on the standard HTTPS port. You'll only be able to do this as long as your have nothing else listening on that port.
You can confirm what ports are in use by using netstat -ant.