Also, in an interview Ken claims that he was gifted his grandaddy from a group of natives he helped out in Northern California who had been back crossing a Purple Afghani brought to Cali by some Vietnam war vets. He also says that "Thai" and "Buddha" were brought back by them and I think he kinda hints that the GDP may have a mix of some of those traits in it's lineage as well.
I've never found any legit source that says it is Purple Erkle x
Big Bud, I don't know if he states this elsewhere or not.
I did my best transcribing the video for you folks.
There is a very popular Purple Erkle x
Big Bud cross in Northern California however but my opinion is that this is more of a Grandaddy knock-off than the factual lineage of the strain, however I have not grown it myself and smoked it maybe on 2-3 occasions from the same grower. It had the "purple" smell but there are so many purple hybrids in Northern California.
EDIT: In the video interview Ken says his GDP is a "daughter" or "sister" of a skunk variety. I would interpret that as his GDP ultimately being a (Purple Afghani x some combination of Buddha and/or Thai varieties) x unknown Skunk variety.