Hello,
Apparently this forum is somewhat dead at this time. It appears that you have a B&W turn over gooseneck hitch installed on the truck. I have the same. As the others were stating, avoid a gooseneck conversion kit for the fifth wheel camper itself. The two best options you have are the B&W Companion Fifth Wheel Hitch, or the Andersen Ultimate Hitch. Both will attach to the gooseneck ball you have. I have an older Andersen steel hitch, and have had no problems with it. The most important thing you will have to consider is the pin weight of the fifth wheel you are considering. You have a 2021 2500 Silverado. Check the payload sticker in the drivers side door sill. It will tell you the cargo capacity you have. This will determine how much total weight you can carry/pull.