But anyway I don't really see the point of having that more specific GPD/GP3 chassis code, it is better to have a generic GP for all of that generation.
Many models had their code splitted (often Toyota) in more precise ones, I don't find that useful: that defeats the purpose of the field which allows to easily find all cars of a generation when the model name is reused across many years.
First generation is GP, and that's enough to me:
https://en.wikipedia.org/wiki/Subaru_Crosstrek#First_generation_(GP;_2012)