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. If you are doing a Network Installation, enter
r to toggle
Recovery Only mode. Else, don't
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.
Press Enter and it should start downloading. Be patient while downloading the files.
Recovery Only turned on, it will only download 1 file
Recovery Only , it will download 11 files.