GPU thread seems to be a confusing name as it sounds like everything happens on the GPU but it's actually not. Maybe it's better to fix it sooner (when we're still relatively small) than later.

Specifically,

1. Even if we're using GPU, a lot of work on the "GPU" thread currently happens on the CPU. For example, shader compilations.
2. Sometimes this thread involves no GPU at all (e.g., when --enable-software-rendering is given, or when Flutter is running on the iOS simulator)

One rename suggestion is to follow Android's naming of RenderThread so Android developers will immediately feel familiar.