kdebase-3.5.9 is missing the following files: /etc/X11/Sessions/kde-3.5 /usr/share/xsessions/kde-3.5.desktop those were present in <=3.5.8 release. gdm cannot use kde session without them. (monolithic kde build)
Created attachment 145711 [details, diff] Fix kdebase-3.5.9.ebuild Remove second src_install() function from the kdebase-3.5.9.ebuild. Contents of the second function occurence has been moved into first fuction occurence.
This problem occur because two functions named src_install() present in kdebase-3.5.9.ebuild file (at the lines #162 and #216). Really only second occurrence execute, and no session files has been created. Attached patch correct this problem (see above)
Oh dear... Coming to a mirror near you, thanks for finding the cause!