Is the minimum GPA required for Graduate school a cut-off criteria or a general guideline?
There's no absolute here—it really depends on the program in question. Some departments are stricter about cutoffs than others. It depends a lot on the number of applications they receive, and how selective they need to be. However, I know that many good departments don't screen on GPA alone. 

However, even if you're GPA is not "elite," that's not the end of the world, so long as you have the research experience and the letters of recommendation to support you. (A student from our department was recently admitted to a top-10 engineering program in the US on the basis of his research experience—his GPA was pedestrian at best.)