@Kueeen1999: First, Science is much more competitive at Canadian schools compared to Arts. McGill has some interesting parameters beyond just straight up ACT/SAT and GPA. They will look at all your ACT sub-scores (26 was required for Sci last year). For more details refer to: https://www.mcgill.ca/applying/requirements/minima/usa.
Once you make that cut, they will typically recompute your average based on the program prerequisites plus your other best academic classes. Canadian schools typically look at the top 4-6 subjects and not the whole GPA. They typically do not look at Grade 9 marks. Furthermore, you have to get A- in ALL prerequisite classes. For Arts it is just English plus a few of your best academic classes. For Sci, it is actually Math (Pre-Calc typcally) + 2 Sciences. McGill also typically judges your unweighted GPA so if you get a lot A- or B+ in AP/IB classes you may not make the cut even with your very good weighted GPA. If you are lucky they will lastly look at your course rigour…
Of course, the cutoffs also change from year to year and there could be a lot more interest in Canadian schools (for some strange reason?) this year. Of course, there is another unknown this year: SAT scores are new. Thus, ACT might be a safer and less uncertain bet.
Finally, the McGill process is NOT that holistic. You have to make every cutoff to get an offer. They require balance and it is much harder to hit all criteria than just most of them… such as what happens at most US schools.