For reasons I won't get into, 24fps is the de facto standard for movies. It's sort of the bare minimum for smooth motion, and anything wanting to look "cinematic" tends to be filmed that way. In fact most content is filmed at 24fps, meaning that content filmed at 30/60fps is quite rare. Sports will be at 60fps for maximum motion smoothness, meanwhile dramas will sometimes film at 30fps, leading to the soap opera effect. In the case of Tokusatsu GAGAGA, it's filmed at 30fps like a soap opera.
There's nothing particularly novel about filming the toku sections at 24fps, but it's one of those details that showed someone cared about authenticity. A lesser production would use 30fps throughout (and there's an argument to be had that mixing framerates isn't a good idea to start with, since it leads to juddering on the 24fps sections as frames get duplicated).