GU10 - Aluminiumised
GZ10 - Dichroic
The GU10 has a bevel around the base but the GZ10 has a square corner. This stops the GZ10 being used in a fitting designed for a GU10 but allows the GU10 to be used in either.
Aluminiumised (GU10) lamps relect heat forward, dichroic (GZ10) lamps don't, so...