I would recommend you to take first the "required courses" based on the CRNA program you are eyeing. And after meeting all the minimum requirements (GRE, CCRN, CRNA Shadowing, Required Courses: Biochem/Chem courses, Letter of Recommendations, etc) except GPA of at least 3.0, then I would recommend retaking some science courses like Anatomy , Physiology Undergraduate courses. I don't think graduate courses would make such impact since your goal would be to increase your GPA (knowing that graduate courses are tougher and hard to find), unless you're confident that you can get at least 3.3 on a graduate course.
Just to clarify things, when I say meeting minimum requirements, it's usually stated on the program's website. For example, on the school that I applied, it was specified to meet at least 2 of the following requirements: GRE>300, GPA >3, Chem courses within 3 years of application, ICU exp atleast 1 yr, CRNA Shadowing. Meaning I don't necessarily need to have ALL the requirements, that I can still be invited for an interview even if my GPA is not >3.0 , or my GRE is not >300 but I should have at least 2 of them in order to be considered for an interview (Chem course within 3 years of application, CRNA shadowing exp, 1 yr ICU exp ). And the rest will be based on the how you will perform in your interview.
That's why I will recommend you to ensure meeting the minimum requirements and not focusing too much on your GPA. Surely, high GPA is a PLUS but there are other factors that the school are looking into, like the applicant's readiness to study again, the willingness to sacrifice their social life, their devotion on this intense program , financial status and other factors.
Upon doing my research, I saw a bunch of applicants with Science GPA of 4.0, impressive ICU exp, invited to an interview but not accepted to any program.
I personally have 3.23 GPA, GRE 295 3.0 Writing, 1 1/2 yr MICU exp but on my 1st application, I got into the program. So don't lose hope and get petrified or intimidated by other's impressive stats. Good luck!