Sent him an email yesterday about it, this is the response
You are correct, and the bike is not being sold as a GP3. The frame and triple trees are GP3 and there is some debate the swing arm may be as well. The bike is being sold as a legitimate historical motorcycle tied to Ossa and Yankee and the bike builder David Robinson, who did own Ossa in it's final days and is credited for being one of the very first Americans to move to Europe and compete in the highest levels in European motocross, before most Americans new what motocross was.
If you are focusing in on the subtitle of the listing that is designed to use "key words" to attract viewers, and not the actual description of the bike, I can understand your confusion. It is a very rare, unique and historical Ossa and a cool bike! Thanks.