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Gmail is owned by Google so everything goes directly to Google. Google will comply with subpoenas assuming they are correct and meet all of the requirements. Usually they meet whatever time limit is set forth on the subpoena, but I have seen them ask for a minimum of 30 days before with other sections of Google. If you want someone to appear at trial or a deposition for Google then they usually need at least 30 days notice as well. In addition to Google's policy there is the federal Stored Communications Act (SCA), 18 U.S.C. § 2702(a)(1), which "prohibits Internet service providers from producing e-mails in response to a civil discovery subpoena". I'm not sure how much that has been tested yet but there is always a way around those kinds of statutes if someone spends enough time and money.
Any info that they have can be revealed. However, they often require a court order and not just a subpoena if the information is something they consider private such as a social security number. I have heard that they are taking the stance now that everything is private and they only comply with proper government subpoenas or with court order, not just subpoenas in a regular civil case unless the subpoena comes from the local court where the Google headquarters is located.
They have web pages that discusses their general policy on releasing information at https://support.google.com/faqs/answer/6151275?hl=en and at https://www.google.com/transparencyreport/userdatarequests/legalprocess/
There is also an article in the Illinois Bar Journal specifically on the topic of trying to subpoena gmail info at https://www.isba.org/ibj/2015/01/whyyouprobablycan%E2%80%99tsubpoenaemailgoo