GPA should be a result of mastering the material. Assuming that is roughly the case, if you ever go on to use any of the skills you were supposed to learn in school (math, computing, etc) you are asking if having learned those skills would help you perform those skills?

I doubt there will ever be a way to satisfactorily control for other variables when it comes to these sorts of real-life studies (there is a reason the majority of social science isn't reproducible[1])

[1] https://advances.sciencemag.org/content/7/21/eabd1705