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. 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. There should be 11 items to download. Be patient while downloading the files.
Once it finishes downloading, you can proceed to the next part.