Well, math is a bit different from other science and engineering disciplines in that it is very rare for an undergraduate student to get published or even just involved in any sort of meaningful research. Grad school applicants in math are often evaluated purely on their coursework. While it is possible to acquire the same knowledge through independent study, you might have a hard time convincing graduate admission committees that your background is in fact equivalent. The lack of a direct comparison group (other math majors) might work against you as well.
If you pursue the independent study route, your chances might come down to how credible your references are. Be sure to ace the Math Subject GRE because that might be the only objective indicator of your mastery of the undergraduate math curriculum. (The top math programs generally expect applicants to go significantly beyond the undergraduate curriculum, but it's a good place to start.)
As far as GPA is concerned, the absolute minimum for consideration is a 3.0, but GPAs are not created equal. A 3.3 GPA from MIT is different from a 3.3 GPA from the local state university. I don't know where your university falls along this spectrum.