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.