If you are using the c-peptide to see if your body is underproducing insulin or overproducing insulin (insulin resistance) these two extremities will show up in a blood test despite your medication.
Overall, metformin decreases insulin resistance and improves insulin sensitivity, thereby helping the insulin your body still makes work more effectively, which should, in turn, lower your body's insulin requirements, which will lower your c-peptide. However, gliclazide works by increasing the amount of insulin your body makes, which would increase your c-peptide. Whatever the balance of the two drugs you achieve, your c-peptide should be helpful.