Although the default theme of Plasma, Breeze, provides a GTK 2 theme, the script /etc/kde/env/gtk2_rc_files.sh and the config file /usr/share/kde-settings/kde-profile/default/gtkrc-2.0-kde4 set the GTK 2 theme to Adwaita.
Furthermore, with the file ".gtkrc-2.0-kde4" installed in the user directory and the "GTK2_RC_FILES" environment variable pointing to it, changing themes at runtime from System Settings doesn't work. I don't know how GTK 2 theming works but it seems that, like with GTK3, it can get its settings from xsettings (that KDE Plasma supports through xsettingsd). So it seems like the gtkrc file isn't even necessary.