Gatekeeper is a security feature designed to protect your Mac from malware. Whenever you open an application for the first time, Gatekeeper will check that application against a list of apps that Apple has reviewed and approved. This simple defense against malware was introduced in OS X Mountain Lion. You can configure how strict you want Gatekeeper to be.

• Open  > System Preferences > Security & Privacy
• Select the General tab

In the bottom half of the screen, you see the text “Allow apps downloaded from:”. There are two options.

• App Store
• App Store and identified developers

As you may have guessed, the first option allows only applications from the App Store to be installed. The second option allows apps from the App Store and apps from developers checked and approved by Apple.

There used to be a third option, Anywhere, which allows all applications to be installed. That option effectively disabled Gatekeeper. This option has been hidden since macOS Sierra. However, you can get the third option back by disabling the gatekeeper with the following command (which requires your system password).

sudo spctl --master-disable

Disabling Gatekeeper is at your own risk. You should only disable Gatekeeper to install an app from developers you trust and re-enable Gatekeeper as soon as the app is installed. To re-enable Gatekeeper, use the following command.

sudo spctl --master-enable

From that moment on, Gatekeeper is on duty to protect your Mac from malware once again.