Yes Manolis, I did several attempts and follow your suggestion and the information on the provided URL.
I took both/two offered methods with my Raspberry Pi4 (c03112, 4Gb, 32GB-SD with fresh installed 2020-02-13-raspbian-buster containing
"Raspbian GNU/Linux 10 (buster) VERSION_ID="10") and my RTL-SDR.
Let me share my findings:
Method 1. Manually installation.
The "Add repository install manually" resulted in an installation of the GNURadio which could be launched from the GUI.
After installing the RTL-SDR resources I was able to test the test program 'rtl_test" from the terminal window.
I was also able to use the RTL-SDR as a source, but the problem was that the GNURadio source block was not able to connect to the actual RTL-SDR stick.
This is strange for me to understand because the test program rtl_test worked fine so the driver attaching to the USB and hardware seem to be ok.
Method 2. Installing the deb-file.
The installation stopped after a long list of "...failed to install due to dependency problems..."
Yes, this is unfortunately the situation.
Two be sure I did not make any mistake I performed each installation sequence two times after reformatting and installing SD-cards freshly.
Any suggestion to get WBFM music with Pi4 and GNURadio are more then welcome!