While I'm a KDE user, I still use some Gnome appliciations or other applications with a GTK interface (the gimp, synaptic, firefox and thunderbird for example). Their default skin when running under KDE (on my Kubuntu Linux system) was a bit ugly in my humble opinion, not to say it felt like motif uglyness.
Logging in under Gnome and changing the theme there didn't change it under KDE.
The solution was/is gtk-theme-switch2.
This little program lets you preview and set GTK-2.x themes so your
GTK/Gnome applications have a nice skin under KDE.
Get it through your distribution's package system, it should be there.
Under Gentoo I had to emerge gtk-chtheme.
You can download themes from art.gnome.org and www.gnome-look.org.