Follow the links to download for your operating system from here. Windows installation is straightforward. Mac users will find themselves here, and will first need to follow the links under the QGIS Standard heading to download and install the two requirements: GDAL and Matplotlib; then download and install QGIS itself.
Download from here. If you had a previous installation of Gephi on a Mac, you find find that the launch freezes at “Opening Main Window.” If so, in Finder select Go>Go to folder and navigate to /Users/YourUserName/Library/Application Support, substituting your own user name for YourUserName. Delete the old Gephi folder.