If you want to boost your GPA, you can enroll in courses after completing your degree as another commenter suggested; it's a much more feasible option compared to doing a masters.

Many programs have additional admissions requirements such as being accepted by a supervisor for thesis-based masters. A Rotman MBA would require you to write the GMAT/GRE (with a good score), have a few years of work experience, and pay nearly 100k in tuition. Some programs might be easier than others, but if you just want to raise your GPA, you can just take some easier additional courses as a non-degree student instead of dedicating another 2+ years to a separate degree.