This particular article doesn't say that about gatekeeper though - the GP's question is valid:
If that weren’t enough, OS X Mountain Lion is introducing Gatekeeper, which will, by default, limit the apps you can install to apps from the OS X store or verified Apple developers.
I disagree. The operative phrase here is "by default". Which implies that there is some "non-default" option. The relevant question posed by the GP was this: "Is Apple really restricting you to install only app store and Gatekeeper verified apps?" That question is answered by the phrase "by default". Not entirely unambiguous, so why not take a moment to look in to it.
On the very first page of Google search results for "os x gatekeeper" is the official Apple page, with the screenshot of the preference panel:
If that weren’t enough, OS X Mountain Lion is introducing Gatekeeper, which will, by default, limit the apps you can install to apps from the OS X store or verified Apple developers.