GMail doesn't really "leak" information about e-mail addresses, it just displays any public information tied to that address's Google Plus profile.
Once upon a time, Google dedicated a massive amount of resources to launch a social network to compete with Facebook, Google Plus. When G+ launched, one of the "features" was linking your GMail account with your G+ account. G+ didn't really take off (obviously), but all Google employees were "strongly encouraged" to join, and the account you're looking at ([email protected]) belongs to former Google engineer, who does have a public G+ profile. If you go to plus.google.com and search for that e-mail address, it will bring up their profile page. If you know any GMail addresses that don't have public G+ profiles, you can verify their information isn't leaked.
Aside: G+ was the reason why Google Reader was killed #NeverForget
Once upon a time, Google dedicated a massive amount of resources to launch a social network to compete with Facebook, Google Plus. When G+ launched, one of the "features" was linking your GMail account with your G+ account. G+ didn't really take off (obviously), but all Google employees were "strongly encouraged" to join, and the account you're looking at ([email protected]) belongs to former Google engineer, who does have a public G+ profile. If you go to plus.google.com and search for that e-mail address, it will bring up their profile page. If you know any GMail addresses that don't have public G+ profiles, you can verify their information isn't leaked.
Aside: G+ was the reason why Google Reader was killed #NeverForget