To the best of my knowledge, Google is not restricting its search engine. The phrase "gun dealers" gives about 24,000 results. "Guns" under "Froogle" returns about half a million entries, about 24,000 confirmed.

As far as whose paid ads they wish to display, that is entirely up to them, and is in no way censorship. If they wanted to take ads only from those products that have the letter "Q" in them, they could.

In fact, as a private company, if Google did want to filter its search engine to not return any entries about guns, that too, would be their prerogative. And that too, would not be censorship, at least not of the kind the First Amendment refers to, which is government censorship.

Of course, such filters would make search engine users tend to use someone else's search engine.