After cloning/downloading gibMacOS, you'll have to open the the folder where it is (or cd to it if you clone it with git). If you're using macOS or Linux, you should know how to cd to it through your preferred Terminal emulator. If you're a Windows user, follow these instructions to get to the command line inside the folder.
Open Command Prompt/PowerShell and cd to where it is.
OR open the folder where it is, press Files > Open Windows PowerShell
Windows Explorer File Menu
Now that you're inside the command prompt/powershell/terminal, we'll use gibMacOS to download the Recovery HD to be restored:
run python gibMacOS.command -r -v <macos version> and replace <macos version> with your target version:
10.14 or mojave for macOS Mojave release
10.13 or "high sierra" (with quotes) for macOS High Sierra release
For the older releases, just download the .pkg file directly, as they've been removed from the catalog that apple provides: