I suppose you are connecting to the GSAD by using a web browser and not via GMP to the GVM (former known as OpenVAS Manager).
It is still the firewall or to be more specific the AWS double NAT, if you don´t wanna set your hostname correct you may either use a VPN or a Tunnel to connect to the private IP Address where you bound the GSAD.
There is no other way to do it securely …