Gatekeeper

Gatekeeper is an anti-malware feature  of the Mac OS X operating system. It was first included with OS X Mountain Lion. Gatekeeper allows users to restrict which sources they can install applications from in order to reduce the likelihood of inadvertently executing a Trojan horse. There are three options: allow only applications downloaded from the Mac App Store to be launched, allow applications downloaded from the Mac App Store and applications signed by certified Apple developers to be launched, or allow all applications to be launched.

Note
There was a utility called Gatekeeper for the Classic MacOS. It was written by Chris Johnson and was stored at the University of Texas at Austin Computation Center's Macintosh software archive. Other than name and function it has no relationship to the MacOS X program of that name.