The Gtk2Removal1Test that checks for the correct warning messages is always executed when the platform is Linux, because it contains

assumeTrue(PlatformUtil.isLinux());

However, there is not strictly a 1-1 link between the platform being Linux and the glass implementation being GTK.
With the upcoming Headless platform, this test is executed (and fails). The same happens with the Monocle Headless subplatform.
The test needs a better way to detect whether GTK is used or not.