Gmail is an Electronic Communication Service as defined in 18 U.S.C § 2510, meaning its contents are protected under the Stored Communications Act (18 U.S.C. Chapter 121 §§ 2701–2713).
Communications with an AI system do not involve a human so are not protected by ECPA or the SCA and get less protection. This is controversial and some people have called on ECPA/SCA to be extended to cover AI services. That means a warrant would be necessary to get your OpenAI history, not just a subpoena.
In a way it's like someone talking to themselves in the bathroom mirror. It's almost a higher privacy expectation than regular emails. You expect no human to see it at all.
Communications with an AI system do not involve a human so are not protected by ECPA or the SCA and get less protection. This is controversial and some people have called on ECPA/SCA to be extended to cover AI services. That means a warrant would be necessary to get your OpenAI history, not just a subpoena.