You can't use GUCs on VS metal Award flights. Only revenue fare bookings are eligible for GUC upgrades with partner metal (Award flights must be on DL metal booked with Skymiles to be eligible to be upgraded with GUCs).

G class is just the inventory bucket class VS chooses to use to determine whether or not a flight is available to apply a DL GUC upgrade on and so that is the class the GUCs book into on them. AF/KL happen to choose to use the Z revenue fare inventory bucket class to determine eligibility for GUC application.