Set the control knob orientation in GarageBand for iPhone
You can set whether Touch Instrument control knobs respond to linear gestures, circular gestures or both. By default, the orientation is set to Automatic, which lets control knobs respond to both linear and circular gestures.
Set the orientation for control knobs
Open the Settings app, choose GarageBand, then tap Knob Gestures.
Choose the orientation you want to use (Automatic, Linear or Circular).