Apple has just pushed an update to the data used by Gatekeeper, bringing its version number to 139, dated 24 May 2018. As Apple doesn’t provide any details of such updates, we don’t know what this changes, but in the case of Gatekeeper updates it is generally thought to reflect recent revocations of security certificates as a result of their abuse.
You can check whether this update has been installed by opening System Information via About This Mac, and selecting the Installations item under Software.
A full listing of security data file versions is given by LockRattler and SystHist for El Capitan, Sierra and High Sierra, available from Downloads above. If your Mac has not yet installed this update, you can force an update using LockRattler, or at the command line.
I have updated the reference pages here which are accessed directly from LockRattler 4.2 using its Check blog button.