The Google API referenced in the accepted answer is deprecated. As of year 2025 there is another simple method to know own Google id:

• Log in to your Google account if not already
• Go to https://myaccount.google.com/data-and-privacy
• Locate the button Maps Timeline in the History settings, right-click on it and copy its URL
• That URL contains 'gid' parameter which is your Google ID.

Hope this helps the folks looking for their google user id in 2025.