The mutants play a movie of how the future being a wasteland is humans' fault.
Gamo says to a returning Yu to please inform everyone that the wasteland could be avoided.
Values Dissonance: When Princess asks to become the leader of the kids, Gamo objects. Why? Because she is an evil bitch? No. Because she is a woman. And that women are apparently too sensitive to make serious decisions.
To be fair, though, her being an evil bitch is the main reason he objects to her leadership. He added her femininity more as an afterthought than anything else. Also, Japan is traditionally a very patriarchal society and Gamo's opinion no doubt reflects this.
Although, later on, Gamo does say that women in general (not just Princess) cannot be leaders because they are too emotional and sensitive to keep a clear head. None of the other boys object to this and all of the important "ministry" jobs are taken by boys. Again, this mirrors Japan's patriarchal values (not to mention that this manga was written during The '70s, itself very prone to Values Dissonance).
How about the eugenics argument in the mutant leader's speech about how part of what caused the end of the world was how modern medicine prevented the weak from being "weeded out"? You could chalk it up to him just spouting the mutants' obsession about the strong surviving like they do to themselves, but since its used as part of the Green Aesop, it kinda seems like the moral is supposed to be taken at face value.