GaussView is not freeware. It is a proprietary, commercial graphical user interface (GUI) designed to work with the Gaussian computational chemistry package.
Go to System Settings > Privacy & Security . Scroll down to the "Security" section and click Open Anyway next to the GaussView notification. Alternatively, you can clear the quarantine attribute via Terminal: gaussview download mac new
file is downloaded, the process is straightforward but requires one specific system tweak: Install XQuartz: GaussView is not freeware