Gmail now promotes Google Inbox. If Inbox is enabled for your account, you might see this message: "Good news – Inbox by Gmail is enabled for this account. To use it on the web, go to inbox.google.com. You can always use Gmail at mail.google.com."
Gmail also shows a promotional link for Google Inbox at the bottom of the page: "Take me to Inbox".
If you click "x", Gmail hides the link and shows this message: "You dismissed the Inbox link." Click "Don't show this again" to permanently hide the Inbox link.