How to find the gvm for my caravan?

The GVM should be displayed on the Van compliance plate.
If your van doesn't have one you may need to obtain an Engineering certification.

We had an old 70's Golf van which had no Vin or compliance plate, built long before such things were thought of. It did have as was the norm a number written with a welder on the A frame. All that was required in those days.
The tare was on the rego papers and no GTM was given or required.
I guess as long as you don't exceed the capacity of the lowest rated component be it tyres, hitch, axle capacity etc then that is all that is required.

I reckon the GVM would be about 1250 kg, maybe a bit less, most of the older vans had a recommended pay load of about 300kg. It still didn't deter my dad from packing it to the hilt. How it survived is beyond me. Insurance is another issue if overloaded.