You can login to your instances from the admin console according to this : https://cloud.google.com/compute/docs/instances/connecting-t...
So Google has a way to login to your instances...
Of course the question is basically moot, because unless they have some sort of third party append-only log of actions they perform on the hypervisors, how would anyone ever know? Yes, AMD this and Intel that, but since Google is building their own computers, and since it's close to impossible to verify that you are in fact running in the secure enclave ... then you should assume you aren't.
Naturally, if you come up with a structure where you can incentivize Google to remain honest (eg. somehow make it evident to the world if they access or tamper with your stuff), then it becomes safe to delegate running VMs to them.
Again, of course, the same problem comes up if you try to do it in-house. How do you verify the security staff at your data center is honest? CCTV? Who watches the watchers?
The software would run your VM, and provide some kind of API which your VM could query to be sure it was running in a secure enclave, managed by Intel's signed software. The result of the API could be signed with Intel's key.
But SGX has been broken multiple times, and because people love breaking SGX because it's such a "all the security eggs in one basket" design, it will virtually certainly be broken again in the future.
Furthermore, SGX is reasonably disliked, inasmuch as people consider its equivalence to the security and boundary implications of the Management Engine.
I should note that this by itself is a fairly hilarious proposition.