Gizmoで円弧を書く。

DrawGizmoArc.cs

public static void DrawGizmoArc( Vector3 p, Vector3 up, Vector3 forward, float angle, float radius ) {
#if UNITY_EDITOR
    var oldColor = UnityEditor.Handles.color;
    UnityEditor.Handles.color = Color.red;

    UnityEditor.Handles.DrawWireArc( p, up, Quaternion.AngleAxis( -angle * 0.5f, up ) * forward, angle, radius );
    UnityEditor.Handles.DrawLine( p, p + Quaternion.AngleAxis( -angle * 0.5f, up ) * forward * radius );
    UnityEditor.Handles.DrawLine( p, p + Quaternion.AngleAxis( angle * 0.5f, up ) * forward * radius );

    UnityEditor.Handles.color = oldColor;
#endif
}