Retreating Blade Stall, P-Factor, and Gyroscopic Precession
Related, especially footnote 2: What is the influence of yaw on P-factor?
The answer to your specific question is that when you are talking about the effect of P-factor on a prop, you aren't talking about actually moving the prop disk in pitch or yaw.  You are applying a rudder force as needed to prevent that movement, so there's no gyroscopic effect to consider.
Whenever the prop disk is actually pitching or yawing-- regardless of whether the cause of the movement is P-factor or something else-- then you have to consider the gyroscopic effect, which makes the force act as if it is applied to the blade disk at a point 90 degrees further along in the direction of rotation.
In helicopters, the individual blades can be moving in a way that creates the same gyroscopic effect as pitching or rolling the entire blade disk, even when the blade disk as a whole is not pitching or rolling.  This is due to the way the blade is hinged.  This wouldn't happen with a rigid rotor disk.  So that's why-- at least with a hinged blade system-- the P-factor effect acts 90 degrees further along in the direction of rotation from the point where it is actually applied, even when the body of the helicopter isn't pitching or rolling, and even when the circular path traced by any given rotor blade tip is remaining constant over time.