Would be nice to have and I can't think of any reason not to expose WebGPU to a service worker considering Webgl is accessible.

It's only exposed to the Window and DedicatedWorkers at the moment. Reference: https://gpuweb.github.io/gpuweb/#navigator-gpu
[Exposed=(Window, DedicatedWorker), SecureContext]

Similar issue regarding exposing webGPU to a dedicated worker: