I'll fill in the details for the curious. You receive an email from `usernotice@google.com` with this body:

  Dear Google user,

  Google received and responded to legal process issued by the United States Department of Justice (<FEDERAL DISTRICT>) compelling the release of information related to your Google account. A court order previously prohibited Google from notifying you of the legal process. We are now permitted to disclose the receipt of the legal process to you. The agency reference number or case number on the legal process is <DISTRICT COURT CASE NUMBER>.

  For more information about how Google handles legal process, view our transparency report at http://www.google.com/transparencyreport/userdatarequests/legalprocess/.

  Google is not in a position to provide you with legal advice or discuss the substance of the legal process. If you have other questions regarding this matter, you may wish to contact an attorney.

  Please reply to this email and/or include the case identification number located in the subject line in any further communications regarding this matter.

  Regards, 
  Legal Investigations Support
  Google LLC

In the case of a Federal Grand Jury subpoena, the Government will request a delayed notification of 1 or 2 years to prevent the service provider (i.e. Google) from notifying you. A subpoena only provides generalized records and not content. So things like billing information, login records, etc.. are fair game. The government is required to obtain a search warrant for actual data (i.e. your emails). Typically, Google will not notify you about executed search warrants against your account.

There actually was no e-mail, I just re-checked. I did a search for:

- usernotice@google.com - Legal Investigations Support - Google received and responded to legal process

Nothing from any of those =( It's been years, but there were two search warrants executed and both are in my discovery, but I was not notified of either via Google directly.