I’ve tried to install GVM 20.08 from source with CentOS 8 and had many problems with dependencies, permissions and redis server. After the installation a had the same error messages like you (maybe a little different).
I decided to use the OpenVAS Packaging Project and installed everything in an hour. I only had problems with feed sync, because of a bad Internet connection. Everything else went smoothly.
I wrote everything down (German only) to remember the steps I made for the next attempt.