Visible on your content & to people who interact with you
What does it refer to for accounts without Google Plus?
How's it possible that others can see the visible information?
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From that same page:
Changes you make here show up across Google services like Drive, Photos, Google+ & others.
Your gender and/or birthday could be visible to anyone clicking on your "About" section from any of those services (if you've set them to Public). Blogger also sometimes has an About link for the blog's author.