Then why would the Nvidia blog post on GPP contain the following two paragraphs?
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The GeForce Partner Program is designed to ensure that gamers have full transparency into the GPU platform and software they’re being sold, and can confidently select products that carry the NVIDIA GeForce promise.
This transparency is only possible when NVIDIA brands and partner brands are consistent. So the new program means that we’ll be promoting our GPP partner brands across the web, on social media, at events and more. And GPP partners will get early access to our latest innovations, and work closely with our engineering team to bring the newest technologies to gamers
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Really it comes down to branding. Nvidia want to push the premium brand consistently - you might pay a little more but you get the premium drivers, hardware, etc. They want to stand out more from AMD who imo fell right into Nvidia's trap with their branding response of open/freedom. AMD instead went for a good vs evil thing, and an appeal to the opensource community.
Now on forums like this that works great, there are plenty of gpu fanboys who are emotionally involved with their favourite gpu manufacturer. However most buyers aren't on forums like these, and don't have an emotional attachment to gpu brands. They just want something that works, and that's what Nvidia's "premium" branding says. AMD on the other hand makes you think of people with beards running linux fighting the system, exactly what most buyers of gpu's are not.
Hence the GPP just makes Nvidia slightly more appealing to the masses, where AMD becomes slightly more appealing to the few. It doesn't stop anyone buying either make of gpu. It's just marketing, and as always with marketing Nvidia have probably done it right and AMD have managed to mess it up.