What is the definition of "geometric pitch angle" for a propeller?
From a variety of online sources (e.g. 1 and 2), the geometric pitch angle is simply the incidence angle between blade reference chord and the plane of rotation. 

This makes sense from the definition of blade pitch, which is the distance that a point on the prop would move forward in one rotation, if it's gripping in a solid medium. The conversion from pitch to pitch angle, therefore, has nothing to do with aerodynamics (i.e. lift) and is geometric only.