That's not a proportional/gain adjustment, it is a servo problem. The two little screws that control proportional/gain affect the yaw of the helicopter (the direction that the nose points left or right, rotating in the plane of the main rotor blades). If you are leaning over to one side or the other, it's the adjustment of the swashplate that is the problem. You probably need to disconnect the linkage from the servo coming out of the right side of the helicopter and screw it out a turn or two (or more - to make it longer and push up the right side of the swashplate so it is parallel with the main rotor blade plane/perpendicular to the shaft).