They are different genres though. Gigantor and Big O were all magical and mystical in origin and Gundam was the first to make it all based in "reality". Or try to place some science around it. It made the Gundams and mobile suits tools for countries and armies and not just some dude's personal thing. Its like the difference between the matrix and something like Heat...both have lots of guns...but one is rooted in reality and one is not.
Also, just because things were covered by the news and were in the public eye doesn't mean they had greater cultural impact. Akira/Ghost In the Shell/Gundam we not only popular with the masses, but also have been sources of inspiration for OTHER culturally significant works of film, tv or music.
No ghost in the shell, no matrix.
you can argue with no Gundam/Voltron you have NO sentai series (power rangers, g force ext.)
No akira you most likely only have far future series, and not near future or post-apocalyptic mega cities.
To be culturally significant you have to be influential to the game and the masses. Pokemon IS significant, but people weren't in class drawing the new charmander. Cats were drawing super sayians and giant robots.
some of ya'll have to be born in the mid 90's to not know these things.