To get KDE to display GTK apps well, you really need the ~/.gtkrc-2.0-kde4 theming that kubuntu-default-settings adds. Installing this will likely change your bootup logo to Kubuntu's though, so expect that. (You will obviously need kcm-gtk as well.)
You get all those packages when you install Kubuntu, but if you're just trying to install minimum-level KDE packages you'll have to fix it yourself.
Whenever you change font settings in KDE, it first saves the changes in the normal KDE settings file ~/.kde/share/config/kdeglobals that all KDE and KDE settings-aware programs read, and then also saves basic font settings (autoaliasing, hinting etc) in ~/.fonts.conf, for apps that don't have good KDE integration. What I guess is happening here is that the .fonts.conf settings transfer to your GNOME session. So, delete or rename it.
$ mv ~/.fonts.conf ~/.fonts.conf.bak