Considering the time we saw him last, he was collaborating with mind-rape Kurloz and Lord mothafuckin English, he seems perfectly capable of a little deception.
Valid. However, what does he gain by lying to Rose and saying that Terezi and himself are blackromming it up behind Karkat's back? what is his motive to lie?
Well, first off if Gamzee seriously is working for English, I'll say first that I'm positing that he's the new Doc Scratch equivalent. He's behind the scenes, doing the sneaky chucklevoodoo things. And English/Scratch's plans have been known to revolve around creating conflict pretty heavily. So by capitalizing on the teenagerdom of the trolls and messing with their already tumultuous relationships by putting groups against one another would be a conceivable bit of modus operandi.