I had a email notify me that my icloud was signed into (****@gmail.com) was logged into on a 13” mac book pro
[Re-Titled by Moderator]
You had an Apple ID say example1@gmail.com signed in Mac system preferences , whenever you sign in at www.icloud.com with account credentials a mail is received in web account / as well in IMAP account stating the Apple ID is signed in .
You can create an Apple ID with gmail services , in which web mail address and its password is kept same as Apple ID and password .
See the articles https://support.google.com/accounts/answer/27441?hl=en
Email says my iCloud was signed into a MacBook Pro
