No, this is not a GNOME application, but a Qt 4 app running on GNOME.
Plastique was designed to closely resemble the default style in KDE. With Qt 4.2, we are introducing a new style that will blend better into standard GNOME based desktops as well.
The style is based on the current default style for GNOME, Clearlooks. It will also be available on other platforms.