No
@burnsie.
The ATM is the total allowable weight of the van and the gear you put in it.
You would measure this by putting your van on it own on a weighbridge, sitting on its wheels and the jockey wheel.
The GTM is the weight that would be on the wheels only. This would be measured on a weighbridge by just having the wheels on it, with the coupling either attached to your car or resting on the jockey wheel, but off the weighbridge.
The difference between the two is your ball weight.
The tow ball mass will be given as two numbers. On is unloaded and one is max. This is just the load that the coupling will exert on your tow ball.
So when you load up your van with the 475kg payload, you need to ensure that it is not all sitting on your drawbar. It needs to be balanced mainly above the axles.
I have seen a good drawing if this somewhere that probably explains it better than above.
Key message,
Keep your total weight below the ATM, keep your ball weight below the "Max ball weight"
Your car towing capacity should be greater than your ATM, and your cars max ball weight should be above your vans max ball weight.