'Gattai' as a cycle
Baseball, more generally the concept of sport, long predates the existence of television programming. Through the promotion of sport and the idea of bio-power, the government or those with power can reinforce the concept of gattai. When this idea is constantly being reinforced, eventually creative minds are going to create things that also reinforce this same concept. That is how I believe Go Renjā came to be. While the creators of the show may not have known it, because they have been conditioned over time by the culture surrounding them, they created something that reinforces the ideas of their culture. This representation of culture through art is a recurring theme in history, as all minds are influenced by what surrounds them, even if they are not aware of it. In Japan, the concept of gattai has been reestablished and reinforced throughout history, eventually leading to the creation of the Power Rangers.
This cycle isn't strictly limited to Japan, however. In the American animated series Steven Universe, characters often undergo an act known as "fusion," which involves characters combining to form a bigger, stronger body. While not ever explicitly mentioned to take inspiration from Power Rangers or Go Renja, the show has included many, many references to Japanese television shows. It is clear that inspiration is taken from Japan and the East in Steven Universe. So, in a way, the cycle that exists within Japan may not just be limited to within their country's borders. Shows like Steven Universe have already taken ideas from Japanese television (gattai) and brought them into the American television world. While the cycle of gattai may not be in full swing in the States, it may have recently begun with the introduction of gattai in an American-made television show.