This is a QGIS plugin to work with GeoGig repositories
To install, download the plugin bundle from here. It contains the plugin itself and GeoGig. There is no need to install GeoGig separately.
At the moment, the plugin works only in OSX and Windows.
Unzip the bundle zip file into [your_user_folder]/.qgis2/python/plugins/ (You may need to create this directory)
Open QGIS and make sure that the plugin is enabled, by opening the QGIS Plugin Manager.
Usage is documented here