Yeah but there's other tell-tale signs "I'm going to obsessively watch you from the shadows like a standard supervillain" Gabriel should have picked up on so he wouldn't even need to know Trevor's last name, and I added those while I was raging out and editing the post.
In all fairness, Gabe was watching from the shadows when he already came back as Alucard. During Act III, Trevor and Gabriel were never in the same cutscene until they confronted each other at the end of the game.
And Trevor openly states that he's going to restore his family's legacy in his story mode, even if he never specifies that family is the Belmonts.
Gabriel's killed hundreds of people at this point. Trevor saying Gabriel's very existence "bringing shame upon his family" could mean any number of things. What if Trevor was the son of some warrior who swore to kill Dracula and instead died to him? Or something like that.
Gabriel also states, "I have no interest in getting to know my victims. I don't know your mother, boy."
The worst part of it is that entire game is based around the premise that Gabriel has an all-powerful magic mirror that he can use at any given time to discover all the secrets of the universe, except that it never bothered telling him "Hey dumbfuck, you're trying to kill your own son" until Trevor was already dying.
You got me, here. Why the hell is Gabe gonna keep that all-seeing mirror and never... well, use it? I wonder how much of LoS' story could've been resolved a lot earlier if he just used the damn thing.
AND WHY THE HELL DOES HE EVEN MAKE TREVOR CHANGE HIS NAME TO ALUCARD? There's no rhyme or reason behind it. At all. It's literally just something Konami awkwardly threw into the game to make it seem more like SotN.
That goes along with the whole "Gabriel doesn't know Trevor's name" deal. All purely speculation, but Gabriel didn't know Trevor's name upon making his grave. And as Gabriel was known as Dracula by the populace, and Trevor had the opposite views from Gabe, he called him Alucard. Again, purely speculation.
Also, what hints that Gabriel doesn't know Trevor's name is his line, "I say that because I see you hold one of Gandolfi's weapons in your hand. In your desperation, it seems the Brotherhood has deceived you. Yet I would know the name of a warrior who wields such a weapon."
I know this sounds like a lot of "defending bad writing", but we speculate things on the vague and sometimes bad writing of the original series all the time, so it's not really wrong to do it here.