Known? Sure. If you ask them they'll also probably know about Barney, the Teletubbies, the Flintstones, or even such things as Buddha, the country of Argentina, the sun. That doesn't mean many people are fans/followers, or that they have much popular presence. Simply being known isn't claim to popularity or widespread.
The issue isn't Gundam not being popular relative to it's ratings in Japan, it's of Gundam not being popular relative to other shows in America.
Believe it or not, Barney, the Teletubbies, the Flintstones, Buddhism, Argentina, and the sun are all very popular things. Barney and Teletubbies are still very actively watched by the young, many people still identify themselves as fans of the Flintstones, many people practice Buddhism, and many people are completely fine or happy with Argentina's existence (just because Chavez hates America does not mean everybody hates Chavez, in fact, it might mean more people like Chavez than hate him for his stance on the US).
Conversely, everybody "knows" shows like HIMYM and the Office. Does a very large amount of people watch the show? No. Many people have seen a couple of episodes and like it. However, the amount of hardcore fans/followers that watch every episode and dream about it probably is around the same size as the same group with Gundam. With your logic, those shows obviously must not be popular at all. In fact, with your logic, the only thing popular in America is football.
You need a lot of popularity if you are going to be "known" in the American world. However, with obviously differing definitions of popularity, judging whether or not a series is "popular" or "unpopular" is subjective. However, it is my opinion that some people here are underplaying the popularity of Gundam and are not giving enough credit to the efforts made to localize the series.
Why is it not
more popular?
Gundam was made for teens and young adults. Cartoons in the United States are seen as something for children. When a localization group markets a more mature series for the younger kids, then both groups are not happy: the younger kids don't get it and the more mature group think its too kiddy.
I don't know why American Gundam fans hopelessly cling to this assumption that Gundam is a ginormous blockbuster waiting to happen, if only more people knew about it. The high point with Wing is nearly a decade behind us. Gundam is never going to be that big again, and no one can or will do anything to try to make it big. It's best to just drop these pointless fantasies of things that aren't going to happen.
This. I believe the reason why Wing was more popular was because of it focused more on younger main characters that the other series. G Gundam also did pretty well because it had the over-the-top fighting that DBZ had proven effective in catching kids.