That sounds like a parable created by rich Christians to make themselves feel better.
-My initial thought as well. I agree it's definitely about eternal life or "salvation". I just did some poking around and found the same thing you did in a couple other places stating that there is no contextual evidence for such a "gate" ever existing. I also learned that the phrase "Harder to fit a camel through the eye of a needle" was not an uncommon one during the time of Christ. It was a popular saying among Jews to convey the difficulty of a task. We tend to ascribe all of these parables/metaphors to Christ and forget that he may have learned or heard many of them many times and they could have been common place. He may have used them in new and unique ways but some were certainly just the parlance of his time.... dude.