I'm unsure if this is the proper place to post this question sinse indirectly references a third party source. But heres an question that occured to me after seeing the Marksman Gunslinger archetype being developed by Dreamscarred Press.
If a character who obtains grit through a class (other than the UC gunslinger) that grants grit based on wisdom, then dips Gunslinger Mysterious Stranger archetype gunslinger gaining grit based on charisma, would the girt gained from both classes pool.
probably not though you might look at ninja and see if it specifies.
I believe you pick a single stat to apply.