I have the same error in kali 2020.4 (for raspberry). The gvm-setup is having a bug in the gvm-start, this is the workaround solution :

after you install your gvm (openvas), execute the following commands:
#gvm-setup
#gvm-check-setup (follow its instructions and execute their recomended commands, until it says "It seems like your GVM 20.8.0 installation is OK.")

then open your browser in: https://127.0.0.1:9392

Note: every time you restart kali, you'll have to execute gvm-setup and gvm-check-setup

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NOTE: DON'T use the gvm-start until kali fix it, i tried the following, and only started the openvas webserver:

The error is because the gvm-start command (/usr/bin/gvm-start) tries to find the GSA_PORT parameter in the /etc/default/greenbone-security-assistant, but that file is does not exist. Since, that file is used to set openvas variables. if you create a file and add the variables, the gvm-start command works fine (but does not start gvm):

cd /etc/default/
sudo touch greenbone-security-assistant
sudo nano greenbone-security-assistant
add the following line: (or the port for the gvm webservice)
GSA_PORT=9392
save and close the file, now execute gvm-start

Another method:
#nano /usr/bin/gvm-start
change the PORT parameter for your web port:
#PORT=$(awk [function that sucury detects as exploit] /etc/default/greenbone-security-assistant)
#PORT=9392

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As I said, this only start the web application but no the openvas server, so use the gvm-check-setup until they fix it.

gvm-start script is now fixed in gvm version 20.8.0.2