In order to use the Github version of Gigastructures, download the zip version of the code as seen in the image:
Navigate to Documents/Paradox Interactive/Stellaris/mod/ (or wherever you have your stellaris folder or custom mods folder if you've changed it manually) and unzip the contents of the folder (the zip should contain a folder called Gigastructures-Live-Branch) into your mod folder.
Irony Mod Manager will autogenerate the .mod file needed to locate the mod, so if using that the next steps are not needed, just launch Irony. Irony can be found here.
If using the base paradox launcher, you will then need to create another (text) file called Gigastructures-Live-Branch.mod with the following contents:
path="mod/Gigastructures-Live-Branch"
name="Gigastructural Engineering & More (Github)"
picture="thumbnail.png"
remote_file_id="1121692237"
tags={
"Gameplay"
"Megastructures"
}
supported_version="3.10.*"
Change supported_version to whatever the newest stellaris version is if necessary. Place the .mod file in Documents/Paradox Interactive/Stellaris/mod/. That is alongside the Gigastructures folder with the mod content, not inside it. You will need "file extensions shown" enabled in windows to verify the proper file type.
After that you should be good to go and can find the mod in your launcher of choice. See also the Wiki page on how local mods work.
It is also possible to clone the repository through usual Github methods into the Documents/Paradox Interactive/Stellaris/mod/ folder, create the necessary .mod file or generate it, and keep the mod up to date with Github tools. See for example this page. Getting updates for a local copy is much easier this way.