After installing net-im/session-desktop-bin the desktop entry generated pointed to the wrong path, /opt/Session/session-desktop instead of /opt/session-desktop/session-desktop. Now I looked into the ebuild and found a mixture of hard-coded "/opt/Session/" on the one hand and "${MY_PN}" or "${DESTDIR}" on the other hand. This probably generates the wrong path in /usr/share/applications/session-desktop.desktop.
sorry for late comment , will fix it.
FIXED https://github.com/gentoo/guru/commit/f12c51bfdda8f1f520975608a808571e815b6dfa