There are a number of gym ball plugs on Thingiverse, but no nozzles for pumping them.
This nozzle suits schrader (car type) valves on a bike pump.
Make sure the nozzle is sufficiently hot for the layers to bond.
Print upright. Despite having a hollowed base (to allow space for that spike which presses the valve) the job adhered fine to the heated base on my printer.