Well at least she'd get some vindictive pleasure out of making Bella guess.

"No, not Jack." She had a lot of Hufflepuff friends, but honestly had only really been close with Zelda and Oliver in the long run. Would she even tell Bella if she figured it out? That was the real question, but as soon as Gem started trying to figure it out she already knew the answer; yes, because she was a terrible liar.