I have a 27 act, uw gpa of a 3.5 from 10- 12th grade, I am in IB, and I am an American student.
https://mcgill.ca/applying/files/applying/us_admission_standards_2018_0.pdf
The minimum ACT for 2018 was 28 for Arts. For 2019 it would depend on the number of applications but your 27 ACT may make McGill out of reach. There are also minimum subscores.