GVWR and towing capacity are not the same, just to be clear. My Dodge has a gvwr at about 14K, but towing capacity is somewhere around 22K. Most of a trailer's weight is on the trailer axles, not the truck. The only part of the weight of the trailer that is calculated as part of the truck's GVW is the tongue weight.
Calculating the tongue weight on your gooseneck depends a lot on the position of the axles, etc. To know for sure, you'd have to go to a scale and weigh your truck then hitch up the loaded trailer and weigh the truck without the trailer wheels on the scale. I've heard figures like 15% and 20% of trailer gvw for a gooseneck, but like I said a lot depends on the configuration of the trailer.