Because Gabe has always been in love with Dad, but Dad fucked Mateo first. He feels like Mateo stole his first love from him.
That’s not it
He felt that Mateo who had always had more of a bad boy persona, basically manipulated their father into that relationship. And in fact he kind of did manipulate them into fkng after he slipped something in either their father or Gabe’s drink (can’t recall) and it led them Gabe and the dad to F for the first time. He things it’s fk’d up he did that and resented him for it. But at the same time, Mateo is the one he’s most attracted to and he continuously represses that feeling so it’s converted in negative feelings instead.
Even when he got over the drink thing, he was still mad that Dad was messing around with Mateo and Andrew instead of just him, so no I wasn’t exactly wrong. He was especially more so angry at Mateo, who insisted on lying about being with Jacob even after being caught afterwards in the kitchen, and that hurt him even more. Gabe does have repressed feelings for Mateo, but he’s still mostly salty about the Mateo x Dad situation. After Gabe and Mateo try to makeup and have sex at the pool, afterwards, Mateo makes a joke about how Dad tastes and Gabe gets mad again. So it’s still a sore spot for him.
He never actually got over the drink thing, honestly. It’s always an excuse for him to get upset. Everything about Mateo talking about the dad just reminds him of that and upsets him again. And another reason he’s upset about the dad being with Mateo and Andrew is because he knows what they’re doing is wrong and he feels he shouldn’t be doing that with them. It’s also why he actually never went there with Andrew until one of the most recent chapters.
the most obvious answer? cause its incest and disgusting af. the more detailed answer? he thinks its disgusting but he's also always been attracted to his dad so seeing how easily his brother went into a physical relationship with him made him jealous (tho unaware) so its basically disgust + jealousy. see how he easily admitted to his feelings and got together with his dad when he got drugged, and during thanksgiving. then he gets jealous when he realized mateo was behind the fridge.
This actually is framed better. And then there’s the fact that while he still thinks it’s disgusting and is still angry with Mateo, he also has repressed feelings for him.
Why is Gabe so mad at Mateo?