oh god this is a long one
Overall Gon has been consistently not very confrontational about Killua’s feelings, which is something I’m not sure is good or bad. He’s never pushed Killua to talk about anything, and for Killua that’s good, because he’s a private person, but that also means that there’s underlying major miscommunication and conflict that never gets properly resolved. As far as we know, Gon had no idea about any of Killua’s character arc regarding Illumi — not the trauma Illumi was causing Killua, not the needle, not Killua’s fear of losing Gon, nothing. Killua just doesn’t talk about that stuff. I actually even noticed that since the beginning of the series Gon always lets Killua run the conversation when it comes to personal topics, and never pushes for more. That IS a good thing, he’s careful not to overstep, but that also comes off as uncaring when there’s a very clear issue such as during the Nobunaga confrontation during Yorknew where Killua was Freaking Out and Gon never tries to get to the bottom of what was going on that was causing Killua to attempt to kill himself to get Gon out (all Gon said was that he’s “not allowed to do that,” and that Gon is “supposed to be the one pulling moves like that while Killua is supposed to be the one to keep him under control,” paraphrasing of course. so clearly Gon registered Killua wasn’t acting like he normally would + that can of worms with making Killua play cleanup but that’s a whole other ordeal).
I really do think Gon prior to Chimera Ant was trying to give Killua the space he seemed to need, but that unfortunately is fundamentally incompatible in the long run with someone like Killua who bottles everything (good and bad) up. During Chimera Ant though, I'm not so sure that was the reason for Gon's emotional distance. I feel like, most likely, Gon just didn't want any distractions at the time. There was enough already going on in his head, and so he chose to pretend like he didn't see what was going on. It's not that he didn't care about what Killua was going through, but given the nature of how their relationship works, he probably figured it'd sort itself out anyway. Except, Gon got more distant and less emotionally available, which resulted in closing off the option for Killua to talk to him. That was an important part of their dynamic, Gon never pried, but he was still there to confide in if Killua needed. So as the issues started piling up, there was no way to deal with them. Killua has almost always taken the backseat for the sake of Gon, so he just sucked it up. During the invasion, he was prioritizing the mission over his feelings and worries, which we saw made him hold himself back several times from speaking to Gon/asking Gon questions. The only time he spoke up against Gon during that arc was when Gon had a risk of jeopardizing the mission, and even then he was holding things back.
In that scene, Killua’s way of coping actually bothered Gon, even though he was definitely aware of how Killua operated. Gon is a person prone to emotion and impulse, while Killua tries to keep his emotions hidden. That whole “you have it easy. you’re completely calm, since it has nothing to do with you” thing is Gon playing Killua’s coping strategies against him knowing damn well it’s not that Killua doesn’t care, trying to make Killua to break just like him because Gon had no outlet for his anger. I don’t really blame him for acting that way, It's normal to feel frustrated when someone isn't outwardly as emotional about something as you are, but it was definitely a comment made with the intent to hurt.
The thing with Gon and Killua’s relationship is that at the beginning it works so well, but over time molds into something irreparably damaged, because the cracks just keep adding without filling them. It may seem like their relationship only started crumbling because of Chimera Ant, but problems have always been present, such as Killua bending to Gon's will and putting him on a pedestal. Gon does show he cares greatly about Killua throughout the series, but he was perfectly content to keep that uneven dynamic (which we know he was aware of, through him telling Meleoron during CA that "killua won't like it" but he can "insist on having it his way and killua will give in"). Gon obviously doesn't want Killua to do anything like die for his sake, that's explicitly clear from their argument about the Nobunaga situation, but he is willingly to let/push Killua to do things that are physically (i.e. destroying his hands in dodgeball, which Gon had known Killua was doing before it was revealed to the rest of the room, or asking Killua to go longer with his hands injured so that he could heal Genthru and his two stooges) or mentally (too many examples lol) uncomfortable.
Neither one of them are necessarily the problem here, I know it’s common to blame Gon for everything but for the most part he was just acting self-centered as kids often are. He was quite selfish, but that doesn’t mean he’s evil or that he didn’t value Killua. It’s just that barely anyone ever put their foot down to try and explain how that was a problem (outside of like, Tsezguerra in GI telling Gon he was being selfish and putting Killua in danger by trying to pick a fight with Genthru while Killua was injured — which, I should add, made Gon feel really guilty after it was explained to him). This was the first time either of them had a friend like this, so they were both learning and trying to adapt to each other’s rhythms. With the codependency they had going on though, they ended up creating a pretty nasty mixture of issues.