Apple has pushed an update to the data used by Gatekeeper, bringing its version number to 175, dated 31 July 2019. The last update had taken it to 173. Unfortunately, the installer claims that this update is version 174, but the bundle version is given as 175. This is likely to cause considerable confusion, particularly if you refer to the list of installations!
Apple provides no details as to what changes this update brings, but it is normally expected to include recent revocations of security certificates used in signing software.
You can check whether this update has been installed by opening System Information via About This Mac, and selecting the Installations item under Software, but it will there report it as version 174.
A full listing of security data file versions is given by SilentKnight,
silnite, LockRattler and SystHist for El Capitan, Sierra, High Sierra, Mojave and Catalina, available from their product page. 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 and later using its Check blog button.
Note: at some time on 1 August, Apple seems to have corrected the updater version number to 175, to correspond with the bundle version. Thanks to Al Varnell for noticing this.