Marks are certainly important, at least initially. After a few years, employers are more interested in what experience you have (in particular, your experience at teaching extension subjects in senior high school.) But good marks will help you to get the initial interview. If you are looking at a job in the public school system, I think they weight your uni results 1/3, your practicum report 1/3 and your performance at the interview 1/3, so marks are certainly not the only thing to be concerned about...
My uni doesn't use GPA, so I am not really familiar with that system. Most students who go into maths teaching thesedays come from the bottom quartile of graduating maths students, so if your average mark is anything above average (i.e., you usually get distinctions or better), you will have outclassed about 90% of the competition.
