Why the hell would he suddenly decide to do that? He didn't have any problems constantly mumbling about who he was and what his motives were at every point prior to that scene. And even if he doesn't specifically say that he's Trevor, he straight up tells Gabriel he's there to avenge the family name that Gabriel tarnished. Does Gabriel honestly not have the slightest clue who this guy who single-handedly infiltrated his castle and uses the exact same skill set as him might be? Isn't he just a teensy bit suspicious when he's shown watching Trevor from far away and scheming to himself? Doesn't his omnipresent magic mirror thingie make him aware of who Trevor is by default?
It's been years since he was gabriel though. It's been Trevor's entire life, and some of Simon's. he abandoned his former identity and just decided to forget all about it.
the brotherhood never stopped sending people after him though, which is why you still find dead knights everywhere. all because he uses one of Gandolfi's weapons doesnt mean much, he's obviously just a bit more special than the other rank and file knights he's fought and killed.
Revenge for his mother's murder? He's killed lots of people. It's not like he keeps count. And trevor was being very obtuse as well. he didn't outright reveal who he was and who he was avenging, and Gabe never cared enough to look into the mirror.
TL;DR both Gabe and Trevor are hardheaded dense motherfuckers, and never bothered to "get to know" each other because they were too busy being at each other's throats.
Ok, so the whole misunderstanding hinges on every character acting like inconceivable morons for no reason.
more or less. Gabe always was a hard headed stubborn person, and trevor is his carbon copy, right down to the brooding.
At the beginning of the game (which chronologically takes place after everything else, but whatever), we see Gabriel has absolutely no qualms with trying to murder the shit out of Sypha (his daughter-in-law) and Simon (his grandson) to punish Trevor.
Gabriel is just an asshole and a retard. he didn't know Sypha and Simon lived where they did. He didn't even know Trevor had any family. But because he decided that as revenge he'd burn down and massacre a brotherhood village, he perpetuated the cycle by killing trevor's wife and setting his grandson on the path of vengeance.
Do mind though, that when you encounter him as Simon/Alucard, he isn't so quick to just come to blows. he even tailed Simon and Alucard. That said, he realizes the Belmont family is just cursed to keep the cycle going so he claims he's decided to just end the bloodline alltogether. But he doesn't go all out against Simon and Alucard. hell, half the fight is him possessing Simon to attack Alucard, and the other is him just doing the disappearing reappearing fire ball strat, where against Trevor he went all out no holds barred. and despite not having actually died when Simon impales him, he still goes away and his castle crumbles, only to eventually rise again, queing Alucard's master plan as shown in LoS2.
Then that ending cutscene in Trevor's story happened and I was just like "Wait, how?"
because they decided to tell the story backwards. basically because they wanted the reveal that Alucard was Trevor to be sort of a surprise. Despite everyone figuring it out before the game was even out.
That said I thought it was done well. gave me some chills when they zoom out on Trevor's coffin and it says "Alucard". And the ending with Gabe trying to revive trevor and failing I thought were well done
basically, i enjoyed it for the opposite of you. I loved the plot and setting, the gameplay not so much