GNX turbo vs GN/T-Type Turbo

GNX Turbos had a ceramic impeller in order to spool much quicker - they also had a foreign particle trap on the exhaust housing to isolate possible contaminants that would otherwise damage that ceramic impeller. HTH. i am not sure if the overall size of the exhaust or compressor wheels were at all different though.
Don't forget the special low-drag dynamic turbine shaft seals that, along with the ceramic impeller, greatly reduce spool-up time / turbo lag.

As Royal-T mentioned, most visible (unless it's off the car) is the contamination trap on the housing with the screw-in "drain" cap.