For JBL Quantum 100/200/300 models, they do not offer Game-Chat audio dial function.
To get the Game-Chat dial feature, you need JBL Quantum 400, and up, then you can adjust game-chat balance by rotating the dial-up and down.

For models that support Game-Chat dial, when you rotate the dial, you will feel a notch that indicates 50% game and 50% chat. Or you can see the status on the top bar of the JBL QuantumENGINE.