Seen the finale; perhaps the most striking victory of Gentaro over Gamou isn't physical (the Fourze/ Sagittarius fight), but rather the moral fight:
- Gamou was a self entitled bastard who had a very high opinion of himself, seeing himslef as a sun, and as a chosen man; and the behavious of his followers ( especially Scorpio, Leo and Libra) made it even worse, since it confirmed that fact: when Gentaro points out that it's friendship that made Libra sacrifice himself for Gamou, Gamou answers that Libra was merely a planet who did what he had to do to his sun; and given Libra's persoanlity, Gamou had a point there; however, even if he refused to admit it, he wanted some companionship; hence his sadness when he had to kill Emoto, because he wished to have him as a companion during his journey, and the next arc with dark Yuki; Yuki and Emoto were likely the only people he felt worthy of being with him
- As opposite, Gentaro values the power of friendship, even when friendship brings conflict (it's especially true with his relation with Ryuusei/Meteor), but Gentaro finds his strength thanks to the fact he feels he has friends on his side.
What happened when Gentaro defeated Gamou? for the first time, Gamou had to see his failure: Gamou has a very high pride, and that pride was broken; and for the first time, he sees Gentaro as someone really worthy; he's like a sun who sees a star brighter than him; and the reason of Gentaro's victory? his belief in friendship. In a way, Gentaro defeated Gamou in the same way he defeated Virgo/ Emoto (even if in the latter case, it was just training); he showed them that his system of values is better than theirs. And the fact Gentaro forgives him, and wants him as a friend completely stun him, so much that he accepts his friendship, and gets probably the only person he can genuinely call a friend = in a very ironic way, his former biggest enemy
And once his pride is broken, Gamou becomes back the little child who wanted to go in space: the way he begs Gentaro to succeed where he failed shows how in a way he looks up to Gentaro, when before, he always looked down on people ; he's sad, but he's only blaming himself, and he shows really affectionate feelings towards his former enemies (and confirmed by the fact he brought back Kengo to life, thanks to the Aquarius Switch), hoping they're gonna do what he couldn't (and validating Rokuro's belief).
In fact, once defeated, and stripped of his pride, Gamou becomes sympathetic.