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You need to use a different render engine like Redshift, Octane, or V-Ray (Limited)
Arnold GPU hasn't been released yet, when it is released, you'll be able to render on the GPU. But the current versions of Arnold only render from the CPU.
You can try to sign up for the private beta of Arnold GPU here: https://feedback.autodesk.com/key/ARN0LD
Not yet,see example of beta tests[ https://www.instagram.com/p/Bs4FXYgHOl7/ , https://www.instagram.com/p/Bu4QUjMFeHD/ ] you can sign up for testing here https://feedback.autodesk.com/key/ARN0LD