Earlier quoted context omitted.
I've deployed a large Office 365 system. It has the capability built in to do ediscovery searches across any item in the system, and can do so without the knowledge of the end user. The organization I'm familiar with audits employee emails that meet certain criteria. Google has e same capability. Providing a "back door" to those systems is a simple as defining a user role.
I'm not sure how large your "large" Office 365 system is, but I'd hazard a guess it's half a dozen orders of magnitude smaller in size and complexity than Google's systems.
But again, Google offers a similar functionality to enterprise Google Apps customers: Google Vault. (https://support.google.com/vault/answer/2462365?hl=en)
Also, for criminal matters law enforcement has been able to obtain search history from Google with a search warrant. So they have the ability to search search history, which is a dataset several orders of magnitude larger in size & scope than GMail.
If you seriously believe that Google doesn't have the capability to perform discovery on the public facing GMail, that's fine, but the capabilities that Google has in production today suggest that your belief is not quite correct.