Apple has just pushed a security update which is new to Catalina systems: Gatekeeper Compatibility Data 1.0. Several other Catalina users have now confirmed this is being pushed generally.
This is a recommended update which is only pushed to Catalina systems, and is small at just 7108 KB. It has no equivalent, as far as I know, for Mojave or earlier, and may contain data concerning notarized software, which has been the subject of speculation for some time.
Both LockRattler and SilentKnight (and its command tool sibling silnite) should offer it as an available update, although they currently don’t register this particular update, as it appears to be the first. It is recorded in two of SystHist’s columns, but not in that at the right. This is because it doesn’t drop a BOM in the list of installations, something which appears to changed in recent Gatekeeper updates too. It also appears in the Installations listing in System Information.
As far as I can tell, this update provides a new Gatekeeper database file contained in the gkopaque.db database in the resources folder of /var/db/gkopaque.bundle, which is the main Gatekeeper database. However, in regular Gatekeeper updates, when that database is replaced its version number is updated. On this occasion, the version number remains unchanged.
(Updated at 1930 UTC 11 October 2019.)