Currently the GPU market consists of companies that have been working on GPUs for decades. Companies that are starting right now have failed to design GPUs. This includes very big players, like Intel, Apple and Huawei. This is likely because all the important ways to produce GPUs have been patented, and there's only so many ways to create an efficient GPU. Even if you try to create your own GPU by poaching engineers from existing GPU companies (like Apple tried), those engieers are only familiar with methods that were already patented and you have no way around this even if you have Apple's tremendous resources to throw at the problem. So the only way to create an open source GPU is to use patents that already expired. Such a GPU might be 15-20 years behind current technology, but that's still a GPU.