Now that I look up what Kyōko is actually chanting... basically, it's a catchy closing part of the Heart Sutra. Here, if you want to hear what it sounds like.

I do wonder how they end up with "gya" and not just "ga" though, given that the original sounds like "gah-teh". 🤔 I guess it sounds better when you drag the syllable out in a prayer session? "Gaaaaaaaaaaaaaaaaaa" vs. "Giiiiiiiiiiiiaaaaaaaaaaaa".

By the way, both Yuyuko and Kyōko (obviously) say this only to Yōmu.