I was under the impression that between them, nV and AMD hold sufficient IP to prevent any other party from getting into the GPU market.
Don't Intel license some of AMD's IP in order to make their iGPUs? And presumably now their dGPUs.
I don't think anyone without the $billions to license those IP would be making any kind of modern PC GPU.
I guess they could go with PowerVR for something (much) less powerful.
AFAIK the only players that have a snowball's chance of going back to making gaming GPUs are Matrox (still alive making niche cards) and VIA (owns S3 IP, plans to make some iGPU for Zhaoxin IIRC).
Both IIRC have some kind of IP sharing agreement with AMD but the chances of them woking on GPUs are approximately the same of Texas Instrument resurrecting the Cyrix brand to compete with Intel and AMD.
That said, Zhaoxin is the actual concrete example for nearly impossible things happening in the semiconductor world, so never say never...