ebuild media-video/mkvtoolnix-11.0.0 with USE="qt5" does not pull in qt-multimedia as dep so the mkvtoolnix-gui is (silently) not installed (it should be installed by USE="qt5") manual installation of dev-qt/qtmultimedia and re-emerging of media-video/mkvtoolnix-11.0.0 installs mkvtoolnix-gui. see my forum-post https://forums.gentoo.org/viewtopic-p-8060978.html tested on ~amd64.
Sorry, looks like I missed this when looking at the changelog and it doesn't help that I only use the cli interface. :) Fixed in 11.0.0-r1 in the tree.