Comment 21 for bug 1913775

Those are the packages that are typically installed by default for this specific source package. It is possible that you have installed additional binary packages (such as libgtk-3-dev or libgtk-3-doc), in which case you would need to manually downgrade those, too.

The source package is called gtk+3.0, and a number of binary packages are built out of this source package. What you install on your machine are binary packages, but Launchpad handles source packages.
If you need to know the source package for a given binary package, you can use e.g.:

    apt show libgtk-3-bin | grep Source:

This specific update won't require a reboot, but I'm not sure whether it requires restarting the current wayland session, or if the fix will be picked up by individual client applications.
@seb128 maybe that could be clarified in the test case?