Finding out GCM of vehicle?

[Franco Cozzo]

Hey guys, I'm curious on how I can find out what the max GCM is of my GMC Sierra, they were imported into the country by the Victorian government back in the mid 90s to 2001, I can see a GVM on the compliance tag and a VIN, no GCM?

I'm curious as to if I decide to not go ahead with converting it to a camper van for what ever reason, might chuck a tray on the back and get a 5th wheel hitch on the back for a van to come later.

Thanks much.