GX feature enables EAX emulation , obviously Rage uses OpenAL already so it's no need for this, this is only for older games with EAX, from the times when Creative had a monopoly over the whole sound market with their proprietary API.
http://images.bit-tech.net/content_images/2008/01/asus_xonar_d2x_pci-express_soundcard/gx1.jpg
Green = Enabled
Black = Disabled