First tell me, what login manager are you using ? is it KDM or GDM..... that option of selecting session types if there in KDM .... please login again....
Otherwise, try creating a new file in you home directory called .xinitrc and add this inside it.... exec gnome-session this would make gnome your default display manager.... and when you give $ startx in shell, then also gnome would start ....
please quote your GNOME and KDE version also....