McGill Admissions

what kind of gpa/ scores does an American student need to get into McGill?

It varies by program:
http://www.mcgill.ca/applying/requirements/minima/usa

I’ve seen that but those are the minimums. What gpa and SAT would insure admissions into the Arts Faculty?

From the link listed above:

“Grades that are equal to or better than those listed below do not guarantee admission in 2018.”

You might want to check this thread on CC:
http://talk.qa.collegeconfidential.com/mcgill-university/2008239-whos-applying-to-mcgill-for-fall-of-2018.html#latest