Westy's GU

[the evil twin]

Well you nailed it.
It's ax cfa car..
So with the gmv upgrade, the GCM doesn't change does it??

GCM will most likely be the same as original.
None of the mobs I am aware of do GCM upgrades (but that is only a couple)
GCM upgrades are available but if they are like GVM upgrades in some states they must be done at time of Rego
Lot more engineering involved in changing GCM

Disclaimer, Some states do not apply GCM to light commercials only LR, MR, HR etc sooooo it depends what data is supplied by the manuf but you still can't exceed the GVM upgrade specs.
Some companies specify that GVM upgrade is against GCM some do not.