Historically, Google treated the 404 (page not found) and 410 (gone) server header page status codes as the same. Both meant, the page no longer exists.

Well, that has all changed now. Google is now treating the 410 as “more permanent” than the 404. Yes, this is a minor change but it is likely an important change for webmasters to note.

JohnMu of Google said in a Google Webmaster Help thread:

So if you never ever will have a page return on a specific URL, then 410 it. But if you never will have a page return on a specific URL, then 404 it.

Forum discussion at Google Webmaster Help.