Run the gibMacOS.bat (Windows) or gibMacOS.command (macOS, with right-click open) inside the downloaded or cloned gibMacOS folder
A Command Prompt window (Windows) or Terminal window (macOS) should appear. Enter
r to toggle
Recovery Only mode.
It should refresh the list after you toggle the mode. From there, you should see a list of macOS versions. Enter the number in front of the latest version (by checking the date) of the highest supported macOS version of you system which mentioned in Requirements. You need to choose the one with FULL INSTALL.
Press Enter and it should start downloading. There should be only 1 item to download. Be patient while downloading the files.
Once it finishes downloading, you can proceed to the next part.