What is the minimum GPA requirement to study PHD in Canada and US ?
I don't know about Canada, but in the U.S., most graduate schools require the CS (and other graduate) departments to have a minimum requirement of a 3.0 GPA. This requirement usually concerns only the the last 60 credits (last 2 years) of undergraduate study. Of course, CS departments can have their own higher GPA minimum requirements. But, even when they do, sometimes it's not always written in stone, e.g.
The desirable minimum undergraduate GPA is 3.5/4.0.
Once you meet the minimum GPA requirement, where you got your degree (University of Toronto vs. Clown College) would be more important than the GPA per se.
For graduate courses taken as part of a master's degree, anything less than a 3.0 GPA certainly would look bad since most U.S. grad schools require a minimum 3.0 GPA to be maintained while in graduate school. However, I don't most think CS departments have such minimum GPA requirements regarding admissions since, after all, graduate coursework is not required for admission to a PhD program.
For more info, I would suggest looking at some of the CS department websites of the top CS departments: