Use of gyro
I agree with BarracudaHockey. But I'd like to take the answer a little further. Heading hold is a mode that's particularly useful in helicopters. Or in airplanes that are trying to act like helicopters. But HH can give significant problems when you try to control a more conventional airplane.
I haven't seen your airplane, so can only analyze it from your description of its behavior. If you could include a couple of photos of the model, that will help. I'm suspecting that you have some combination of small vertical tail or short fuselage length. And maybe some significant amount of wing dihedral or sweep causing excessive rolling reaction to yaw inputs. The design may benefit from more tailoring of the control travels and interconnects. Still, a rate gyro may be the only thing that gives significant help with the takeoff control