The dependency on sys-devel/gdb has been added to kde-base/kdebase-3.5.6-r1 and kde-base/drkonqui-3.5.6 as per Gentoo bug 156758. While this is good idea, the dependency should be (IMHO) conditional on debug or gdb useflag, so it doesn't force ones who don't wanna debug to install quite big gdb. (think of resource-constrained systems) CC'ing Carsten Lohrke, fixer of above mentioned bug, and KDE Team.
Carlo's already on the KDE alias.
It's not an optional dependency. Use the split ebuilds, if you don't want drkonqi.