Hi, author here! Agh I was intending for the project to fly under the radar for a few more days before making the announcement and blog post (please look/upvote that when you see it haha :)
But since this is starting I'm happy to chat. Nice to see the interest here!
wgpu is an implementation of the WebGPU API, so it's basically an alternative to Dawn.
gpu.cpp is one level up - it's implemented using the WebGPU API, not an implementation of the WebGPU API. In theory it should work with both wgpu and dawn but in practice you find there's enough differences it takes some conditional branching + testing to support both.
Having both wgpu and dawn support would be nice and I think we'll get there in the coming months but for faster early iteration I wanted to keep things simple for now. There's implementation + maintenance + testing overhead that you start to have to carry around so it isn't free.
But since this is starting I'm happy to chat. Nice to see the interest here!