I think you mean Gross Vehicle Weight, not Grand Vehicle Weight. That is the maximum permissible all-up weight of the car when it's fully laden (including the noseweight, if hitched up to the caravan). Subtract the Gross Vehicle Weight from the Gross Train Weight (which you haven't mentioned) and you have the maximum permissible towload, which is the maximum allowable total axle load of the caravan.
With reference to the caravan, the only significance that kerbweight has, is to calculate the weight ratio. That is the total maximum weight of the caravan, divided by the kerbweight of the car.
The absolute maximum permissible all-up weight of the caravan (usually referred to as its MTPLM, sometimes MAW, although there are slight differences in definition there) is the specified towload limit for the car plus the noseweight, or the kerbweight of the car if you passed your driving test after the 1st January 1997, whichever is the lower.
However, it is generally accepted that the ideal maximum weight of the caravan should not exceed 85% of the kerbweight of the car, unless the specified towload limit for the car is even lower.
As clear as mud now? Sorry, but I don't know how to put it any better.