Your public health experience doesnt necessarily have to be with a public health agency. Remember that public health is very broad, and can be applied to many different work places. I work at a community resource centre who helps individuals who have no access to secure housing or food, and thats definitely along the lines of Social Determinants of Health. A friend got into an MPH program and works as a Research Interviewer and she was able to relate that to Public Health policies. Regardless, whatever experience you have (which is what a lot of universities are looking for), make sure you know how it can relate to Public Health, and play that up when you're writing your statement of intent, especially with a low GPA. I would suggest to take classes that will raise your last 2 years GPA. A lot of schools are looking for a B+ at least. And getting in with a 3.0 is possible, as long as your statement and references are strong.
Hope this helps.