Khronos Group, the ones behind the development of the Vulkan API, has listed the Google Games Platform Gen 1 aka Stadia as being powered by a custom AMD datacenter GPU based on the GCN 1.5 architecture in their Vulkan Conformant Products page. Google Stadia's GPU is said to be a custom Vega 56 with 56 CUs and a memory bandwidth of 484 GBps. The platform is based on Google's customized Debian Linux and SDK, and uses Vulkan as the graphics API of choice.
https://www.notebookcheck.net/Google-Stadia-uses-a-custom-AMD-Vega-56-GCN-1-5-GPU.421131.0.html