I was informed quite recently by a UK Norton parts supplier that the GPM manufactured pistons are now the type generally supplied as genuine parts by Andover Norton, as the company (Federal Mogul) that supplied AE Hepolite pistons (I guess you meant Hepolite Joe?) will only supply in batches of 5000 (of each size I believe?) so I would expect the GPM type to be a good quality although I haven't used any myself yet.
There are still supplies of Hepolite pistons available although I expect they could become harder to get hold of in the future.
I've used GPMs in my 750 without problem, but did find that they really need ring compressors to fit.
I've never had difficulty fitting Hepolites by hand but the GPMs have a cast oil scraper ring in place of the three-piece spring favoured by Hepolite and they can break during assembly even if you think you're being careful