Ok I'm sorry if I'm being dumb but I'm a little confused- in these flashbacks, does Karkat like Terezi or does he like Gamzee? Because he told Terezi not to "tell him" but then Terezi agreed with Gamzee that Karkat liked her. Or was she holding back? Sorry again, I just realized that I didn't really know what was going on.
/It’s okay! It is pretty complicated, that’s my fault.
So, it goes like this.
Karkat has ALWaYS had a crush on Terezi. Even when they were growing up together. And Terezi liked him back, but it was never in their best interests to be together, so they stayed friends(friends who really liked each other, but never made a move, ya’ know?) In high school, Karkat comes out to Terezi about his sexuality(she’s dating Dave at this point, so Karkat tries to dial down his feelings for her out of respect), and tells her he’s starting to feel attracted to Gamzee (high school, various crushes, raging hormones, you know the drill lol). But she’s sworn to secrecy, because for all he knows, Gamzee is straight as a ruler(he’s not HAHA). So, yeah, she knows that Karkat has deep feelings for her, and she knows that he has a pretty big crush on Gamzee now too. Unnecessary love traingle/square/polygonorwhatever? Abso-fucking-lutely. HAHAHA.