Low GPA, any chance of grad school for math?
Take the graduate version of abstract algebra. Do incredibly well in it, and ask the prof for a letter.

Hopefully it will read something like the following: "X's performance is extremely strong, and his (her?) homework assignments are flawless. I was astonished to learn that X had previously earned only a C in undergradate abstract algebra; whatever X's weakness was, he/she has left it completely beind him/her."