Firstly, an argument should not live or die solely on its own merits, because arguments are made by humans who do not present perfectly rational arguments. A fundamental part of critical thinking is assessing the credibility of a source, so it's only natural to question Gio's intentions. Even if every factual statement he makes is true -- which it may or may not be -- the questions of how and why he makes them, and what he leaves out, are not irrelevant.

Secondly, it's not distracting from the point to discuss Gio's transmysogyny or his past -- character evidence is a form of evidence! It doesn't get you convicted, but it does help (again) assess the credibility of a source. Additionally, if Gio's spotty past should be ignored, then Hussie's should be as well. It's not right to appeal to the previous animosity towards / behavioural patterns of Hussie but declare Gio exempt from that same treatment.

Thirdly, the transmysogyny claims existed before the topic of the pronouns he used in his post. His exclusive usage of he/him isn't the basis, it's supporting information (not to mention the fact that he also tries to delegitimize Hussie's pronouns in the footnotes. Gio explicitly says that he would use other pronouns if Hussie had them, but that he doesn't think she does because it hasn't been revealed anywhere officially) (which is also an outright lie)

So no, it's not a question of "do you like Gio?", it's a question of "do you trust the narrative Gio is presenting?", and for me at least, that is a no at this point.