Dear naturago91,
If you are below a school's average GMAT, than it is critical your GPA be high. For the schools you list, your GMAT is below average so you might want to retake the test to get your score up closer to the average. Even though you have a good GPA and work experience, it would still put you in a more competitive position to increase your GMAT.
It is your entire application package that results in admission to any school, not just your GPA, GMAT, and years of work experience. You must demonstrate you have what the schools want in an MBA candidate. So, it is possible you could gain admission with a 620 GMAT, but it has greater difficulty than if you had a 670 GMAT. I would encourage you to evaluate your entire application package to determine what makes sense for you.
Good luck,
Lisa