35 U.S.C. 102 has the rules on public disclosures and are discussed in MPEP 2152.
35 U.S.C. 102 (a) NOVELTY; PRIOR ART.—A person shall be entitled to a patent unless—
(1) the claimed invention was patented, described in a printed publication, or in public use, on sale, or otherwise available to the public before the effective filing date of the claimed invention; or...
If it is disclosure you are worried about then as long as the email is intended to be confidential and there is assurance from the email provider (Google) that it will be kept confidential then there is no public disclosure. Even if there is a phase in the terms of service that google or its computers may read your emails sent (say for add targeting), as long as they too are bound to confidentiality in the terms of service, then a public disclosure has not occurred.
You would need to read the terms of service of your gmail account to have a definite answer. Probably better to encrypt your email or use an email service that doesn't scan emails for ad targeting.