The HMAC credential that you'd use with the S3-compatible GCS API, also called the "XML API", does need to be associated with a Google account, but it doesn't need to be the main account of the developer. It can be any Google user account. I suggest creating a separate account and granting it only the permissions it needs. It'd be nice if service accounts (aka robot accounts) could be given HMAC credentials, that's not supported. Service accounts can, however, sign URLs with RSA keys.

As another option, you can continue using the XML API and switch out only the auth piece to Google's OAuth system while changing nothing else.

There's a lot more detail available at: https://cloud.google.com/storage/docs/migrating

Disclaimer: I work on Google Cloud Storage.