Average passenger has to be around the 70-80kg mark, I don't think it would expect more than 200kg of load on top of that...

So a guess of GVM could be weight + 80x4 + 200 = 520 on top of kerb weight.

All our corner weights at work are quoted with a driver inside, and GVM tells us what to load the car up to (we weigh it in our weighbridge unloaded to work out how many sandbags to chuck inside)