Google Ad Planner show some site visitors demographic data, like the visitors gender.
Take a peek at Stack Overflow's Google Ad Planner.
As a Web Developer I'm concerned where we are leaking this data to Google. This has anything to do with our use of Google Analytics or it is unrelated?
There is anything we can do to prevent Google to obtain such data?
I have found this site with an answer:
Google Ad Planner combines information from a variety of sources, such as aggregated Google search data, opt-in anonymous Google Analytics data, opt-in external consumer panel data, and other third-party market research. The data is aggregated over millions of users and powered by computer algorithms; it doesn't contain personally-identifiable information.