Karkat had known his feeling for Gamzee had been flushed since the day the two became moirials.
And he also knew the other would never feel the same.
The way Gamzee talked about Tavros and the way his eyes would light up at the mention of the brown-blood's name, that said it all. And Karkat was pretty sure Gamzee knew he didn't have to say anything; that his actions oozed his true feelings towards Tavros.
But what Karkat didn't know was that Gamzee had always known of Karkat's feelings. And it hurt that he could never feel the same way.