Flying a GPS approach is not a PBN certification. Its way more complicated than that.
My reading of it is that while there should be demonstration of capability of being able to doing a GPS approach if thats what you want to fly there is little else relevant to the IMC holder. I am off on my PBN course for work at soon and thats a £20k course in the US in the simulator so I would doubt very much if flying a GPS approach on an IMC renewal cuts the mustard......