From my experience, and what I understand, google has made it so that if you use Inbox, gmail.com will now direct there. Those who don't use inbox are unchanged, and inbox users can turn it off. As an Inbox user I can still access gmail from the app menu, but the url forwards.

Shortly after this happened (last week I think), I saw a few articles saying "Google forces all gmail users to inbox" and "Google Kills Gmail", but I haven't seen any evidence of this, and I really doubt they're going to retire gmail. There's too many people using it, and no reason not to leave both services running if they're using the same backend.