That's pretty neat, I wish they'd publish on how they did this so others could use it. Sounds like another great way to remove friction for the user, always a great thing.
Well...except people generally only appreciate these features when they're allowed explicitly. It's kind of unsettling having a website spontaneously know your activity elsewhere on the browser. Even well-intended, it can come off as tracking data.
It is explicit, apparently FB is using OpenID which is information supplied by you linking the two accounts, completely opt-in. Besides, it's hardly tracking you with this feature, a boolean: is user logged in to Gmail? Yes|No
If yes, user can be verified quickly and reset their password in an easier fashion for them. Facebook is trying to make things smoother and not making you jump into your email to click a link or copy some token id or something. This is good UX.