ok. you wrote that running in level 3 - startx - kde starts ok. this only happens when you change the runlevel. is this correct?
are you sure that you don't have a .xsession file somewhere? this is the only way to put you into gnome env.
check you home and /etc for a xsession file
find . -name "*xsession*" -print
ps. sorry, i am not in slackware now. everything comes from the top of my head.