Are you looking for a loaded weight or an unloaded pin weight ??
I tried a search since I am unfamiliar with weighing the "pin" on a g-neck. I have towed them, but never worried about the weight. There are two figures given % wise of what the "pin" weight should be compared to the actual weight of the load. It is based on weight of load on trailer.
I am more familiar with weighing kingpin on fifth wheel rv's. There is a scale made that hooks onto it and weighs it when it is set down.
I would suggest this as a way to weigh it for you............
Find a public scale and weigh just the truck. Keeping the same weight of the truck, hook up the g-neck and pull just the truck onto the scale keeping it as close to the edge as possible. If the g-neck is loaded, keep the weight of it as far away from the scale as possible. Subtract the two weights to come up with a weight that was put on the truck by the "pin". It may not be an accurate weight, but will keep you in the ballpark. There are various ways of loading the g-neck by shifting around the load to manipulate the pin weight.
There are some business locations around here that will let you use their scales during off peak times. Other than that option, there are scales located at some major truck stops by which tt's weigh themselves. I think there is a charge(not sure..never used..just saw them) and it is kinda' aggrevating since you have to unhook, weigh, hook, weigh......but if it needs to be done...it needs to be done.